OBD-II Code P1304: Ultimate Guide to Manufacturer-Specific Misfire, EGR, and Calibration Faults

What Does P1304 Mean?

P1304 is a manufacturer-specific code, meaning its definition changes depending on your car's make. For Saab, Acura, Honda, Mazda, and Jaguar, it signals a misfire in cylinder 4. For Ford, Lincoln, and Mercury, it indicates an Exhaust Gas Recirculation (EGR) system calibration fault, pointing to a sensor reading that is too high. For Peugeot, Citroen, and Fiat, it highlights an injector programming failure requiring a software relearn. For Hyundai, it indicates a fault with the camshaft position sensor (phase sensor).

Technical definition: The SAE/OBD-II definition for P1304 varies by manufacturer. Common definitions include 'Cylinder 4 Misfire Detected' (Saab, Acura, Honda, Mazda, Jaguar), 'EGR Calibration High' (Ford, Lincoln, Mercury), 'Injector Learning Not Performed' (Citroen, Peugeot, Fiat), and 'Phase Sensor Fault' (Hyundai).

Can I Drive With P1304?

Yes, But With Caution. You can drive, but expect a rough-running engine, poor acceleration, and bad fuel economy. Diagnose it immediately to prevent expensive damage. If the Check Engine Light flashes, pull over safely and shut off the engine. Continuing to drive destroys the catalytic converter in miles, adding $1,200 to $4,000 to your repair bill.

Common Causes

• Faulty Ignition Coil(s) or DI Cassette (Very Common) — A failed ignition coil prevents the fuel-air mixture from igniting in cylinder 4. On Saabs, this indicates the entire Direct Ignition (DI) cassette has failed.
• Worn or Fouled Spark Plugs (Very Common) — A worn electrode or carbon-fouled spark plug prevents a strong spark. Using non-specified plugs (e.g., anything other than NGK in a Saab) directly causes this failure.
• Faulty EGR Valve Position Sensor (DPFE) (Common) — For Ford's 'EGR Calibration High' variation, the DPFE sensor sends incorrect, stuck-high voltage data to the PCM, triggering the code.
• Faulty Fuel Injector (Common) — A clogged or electrically failed fuel injector in cylinder 4 starves the cylinder of fuel, causing a lean misfire.
• Vacuum Leaks (Common) — Cracked vacuum hoses or a leaking intake manifold gasket introduce unmetered air, throwing off the air-fuel mixture and causing misfires or EGR malfunctions.
• Clogged EGR Passages (Less Common) — Carbon buildup blocks the intake manifold EGR passages, preventing the valve from seating correctly and causing a calibration error on Ford vehicles.
• Low Engine Compression (Rare) — Worn piston rings, a burnt valve, or cylinder wall damage cause persistent misfires. An Acura TSB (13-024) notes out-of-round cylinder bores cause low compression and misfires on certain 2013 models.
• Faulty Camshaft Position Sensor (Phase Sensor) (Rare) — On Hyundai models, P1304 points to a failed camshaft position sensor, disrupting ignition and injector timing.
• Wiring or Connector Issues (Rare) — Corroded wiring to an ignition coil, fuel injector, or EGR sensor interrupts the signal. Oil pooling in spark plug wells degrades the coil boot and shorts the connection.
• Faulty Powertrain Control Module (PCM) (Very Rare) — The PCM fails and sends incorrect signals to the ignition coils or injectors. A Mazda TSB (#01-004-17-3317a) confirms DENSO PCMs fail and cause misfire codes.

Symptoms

• Check Engine Light is On or Flashing — The light remains steady for a persistent fault. A flashing light warns of a severe, catalyst-damaging misfire.
• Rough Idle and Engine Vibrations — The engine shakes unevenly when stopped, a direct result of a dead cylinder or an EGR valve stuck open.
• Poor Acceleration and Hesitation — The vehicle stumbles and lacks power when accelerating because cylinder 4 is not contributing or the EGR system is malfunctioning.
• Reduced Fuel Economy — The ECU dumps extra fuel into the engine to compensate for the loss of power, drastically lowering MPG.
• Engine Hard to Start or Stalling — A severe misfire, critical EGR fault, or missing injector calibration prevents the engine from starting or causes it to stall in traffic.
• Smell of Unburnt Fuel from Exhaust — Raw, unburnt fuel passes directly through the misfiring cylinder into the exhaust system, creating a strong gasoline odor.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Replace Ignition Coil(s) or DI Cassette — Parts: $40-$120 per coil; $300-$450 for Saab DI Cassette, Labor: $50-$100, ~0.8 hr book time (DIY)
• Replace Spark Plugs — Parts: $20-$100 (set of 4 or 6), Labor: $60-$250, ~1.2 hr book time (DIY)
• Replace EGR Valve Position Sensor (DPFE) — Parts: $40-$80, Labor: $60-$90, ~0.7 hr book time (Intermediate)
• Clean or Replace EGR Valve — Parts: $10-$200, Labor: $150-$300, ~2.0 hr book time (Intermediate)
• Replace Fuel Injector — Parts: $70-$250 per injector, Labor: $150-$350, ~2.5 hr book time (Advanced)
• Repair a Vacuum Leak — Parts: $10-$50, Labor: $100-$300, ~1.5 hr book time (Intermediate)

DIY vs Professional

• Replace Ignition Coil / DI Cassette — Beginner:
• Replace Spark Plugs — Beginner:
• Replace EGR Valve Position Sensor (DPFE) — Beginner:
• Clean or Replace EGR Valve — Beginner:
• Replace Fuel Injector — Beginner:

Used vs. New Parts: Buying Guide

When a used part is worth it: Buying used electronic parts (ignition coils, DI cassettes, EGR sensors) is highly discouraged due to high failure rates. Only buy used if the vehicle is over 15 years old, the budget is extremely tight, and you perform the labor yourself.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the exact part number matches; superseded parts will not work.
• Avoid parts from rust-belt states; connector corrosion is a primary failure point.
• Used EGR valves are likely as clogged with carbon as the one you are replacing.

Decision logic:

• If The part is an ignition coil, DI cassette, or electronic sensor. → Buy new from an OEM or reputable aftermarket brand (Bosch, Denso). The risk of premature failure is too high.
• If Vehicle is over 15 years old and the new part is over $300. → A used part is a temporary solution, but expect a significantly shorter lifespan.
• If The part is a mechanical component like a bracket or hose. → A used part is acceptable.

Warranty tradeoff: Salvage yard parts carry a 30-day warranty covering only the part, not the repeat labor. New aftermarket parts carry 1-year to lifetime warranties.

Worst-case if a used part fails: $200-$500 in repeat labor costs when the used part fails.

What Happens If You Wait — Timeline

1. 0-1 month: Code sets with a steady Check Engine Light. Misfires cause a slight rough idle. EGR faults show no perceivable symptoms. Vehicle fails emissions testing. (MPG impact: 0-5%% · Added cost: $0-$40 in wasted fuel.)
2. 1-3 months: Misfires cause noticeable hesitation and a rough idle. EGR faults cause engine pinging or knocking under load due to elevated combustion temperatures. (MPG impact: 5-10%% · Added cost: $50-$150 in wasted fuel.)
3. 3-6 months: Raw fuel overheats the catalytic converter's internal ceramic honeycomb, melting it. Exhaust becomes restricted. The Check Engine Light flashes during misfire events. (MPG impact: 10-20%% · Added cost: $1,200 - $2,500 for catalytic converter replacement.)
4. 6+ months: Catastrophic failure. A clogged catalytic converter stalls the engine. Severe misfires cause 'cylinder washdown,' stripping oil from cylinder walls and destroying engine compression. (MPG impact: 20-50% (or car is undrivable)% · Added cost: $3,000 - $7,000+ for engine rebuild and catalytic converter replacement.)

Cost of Not Fixing It

• 0-1 month: Reduced fuel economy (5-15% drop), rough idle, and poor acceleration. Automatic emissions test failure. (Added cost: $20-$60 per month in extra fuel costs.)
• 1-6 months: Unburnt fuel overheats and melts the catalytic converter. EGR faults cause severe carbon buildup and engine pinging. (Added cost: $1,200 - $3,500 for catalytic converter replacement.)
• 6+ months: Severe misfiring causes 'cylinder washdown,' stripping oil from cylinder walls and destroying compression. Long-term EGR faults damage pistons. (Added cost: $3,000 - $7,000+ for major engine repairs or replacement.)

Diagnosis Steps

1. Confirm the Code's Meaning for Your Vehicle
   P1304 is manufacturer-specific. Search your car's make, model, and the code (e.g., 'Ford F-150 P1304') to determine if it means 'misfire,' 'EGR calibration,' 'injector learning,' or 'phase sensor.' This dictates your entire diagnostic path.
   Tools: OBD-II Scanner, Internet Access (Beginner)
2. Visual Inspection
   Check the engine bay for cracked vacuum hoses around the intake manifold and EGR valve. Inspect wiring and connectors for ignition coils, fuel injectors, and EGR components for melting or corrosion. Look for oil leaks from the valve cover gasket fouling the spark plugs.
   Tools: Flashlight (Beginner)
3. Diagnose 'Cylinder 4 Misfire' (Swap Test)
   If P1304 means misfire, swap the ignition coil from cylinder 4 to cylinder 1. Clear the codes and drive. If the code changes to P0301 (Cylinder 1 Misfire), the ignition coil is faulty. If the code remains P1304/P0304, the coil is good.
   Tools: OBD-II Scanner, Basic Socket Set (Intermediate)
4. Inspect Spark Plug
   Remove the spark plug from cylinder 4. Look for heavy carbon buildup, oil fouling, or a cracked insulator. A fuel-soaked plug indicates a spark issue; a white-ashed plug indicates a lean condition or failing injector. Replace if worn.
   Tools: Socket Wrench with Spark Plug Socket (Intermediate)
5. Diagnose 'EGR Calibration High' (Ford)
   Use a scan tool to monitor DPFE sensor voltage with the key on, engine off. It must read 0.5-1.0V. If the voltage is stuck high (4.5-5.0V), replace the sensor. Apply vacuum to the EGR valve with a hand pump; DPFE voltage must rise smoothly.
   Tools: OBD-II Scanner with Live Data, Multimeter, Hand Vacuum Pump (Advanced)
6. Address 'Injector Learning' (Peugeot/Citroen/Fiat)
   This code means the ECU lacks calibration data for an injector. You must use a compatible diagnostic tool (DiagBox, Lexia) to perform the 'injector quantity calibration' routine. Ensure engine and intake air temperatures meet the software's strict prerequisites (usually >30°C).
   Tools: Advanced Diagnostic Scan Tool (Brand-Specific) (Advanced)
7. Test Fuel Injector Electrical Health
   Disconnect the cylinder 4 injector connector. Set a multimeter to Ohms (Ω) and probe the two injector pins. High-impedance injectors must read 11-16 Ohms. An open (infinite) or shorted (near zero) reading confirms a dead injector.
   Tools: Multimeter (Advanced)
8. Verify Injector Pulse with a Noid Light
   Plug a noid light into the disconnected fuel injector wiring harness. Crank the engine. The light must flash repeatedly. If it stays off or solid, the wiring or PCM has failed.
   Tools: Noid Light Set (Advanced)
9. Perform an Engine Compression Test
   If ignition and fuel components pass, test compression. A healthy engine reads 125-175 PSI per cylinder, with less than 15% variation. Low pressure on cylinder 4 confirms internal engine damage (valves or rings).
   Tools: Compression Tester Kit, Basic Socket Set (Advanced)
10. Test Fuel Pressure
    Connect a fuel pressure gauge to the fuel rail test port. Turn the key on, engine off. Pressure must meet manufacturer specs (typically 35-65 PSI). Low pressure causes lean misfires.
    Tools: Fuel Pressure Gauge Kit (Advanced)
11. Test Camshaft Position Sensor (Hyundai)
    For a Hyundai 'Phase Sensor' fault, back-probe the sensor connector with the ignition on. Pass a metal wrench in front of the sensor; the signal voltage must drop from 5V to 0V and return to 5V. If it doesn't switch, replace the sensor.
    Tools: Multimeter, Back-Probe Kit (Advanced)
12. Cylinder Leak-Down Test
    If compression is low, pressurize the cylinder with compressed air. A leak over 20% indicates failure. Hissing from the tailpipe means a bad exhaust valve; from the throttle body means a bad intake valve; from the oil cap means worn piston rings.
    Tools: Cylinder Leak-Down Tester, Air Compressor, Breaker Bar and Socket (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 170-210°F (77-99°C) (The engine is fully warmed up. The ECU diagnoses misfires and EGR faults once the system enters closed-loop operation.)
• Engine RPM: 1500-3000 RPM (The ECU detects misfires during steady-state cruising or under light to moderate acceleration, rarely at idle.)
• Engine Load: 25-70% (The fault sets when the engine is under moderate load, maximizing EGR system activity and making misfires highly apparent.)
• Vehicle Speed: 40-65 mph (64-104 km/h) (Corresponds with steady highway driving conditions where the ECU runs its diagnostic monitors.)

Related Codes

• P0304 — The generic code for 'Cylinder 4 Misfire'. If P1304 appears alongside P0304, it confirms your manufacturer defines P1304 as a misfire, reinforcing the need to check spark, fuel, and compression.
• P0300 — Indicates 'Random/Multiple Cylinder Misfire'. Seeing this with P1304 proves the issue is not isolated to cylinder 4. Diagnose systemic issues like vacuum leaks or low fuel pressure.
• P0401 — Indicates 'EGR Flow Insufficient'. On Fords, seeing this with P1304 ('EGR Calibration High') proves the EGR passages are physically clogged with carbon and the DPFE sensor is likely damaged.
• P1312 / P1334 — Saab-specific codes for misfires on cylinders 1/2 and 3/4. Seeing these with P1304 guarantees the entire Direct Ignition (DI) cassette has failed.

Climate & Environmental Factors

• Cold Weather: Low temperatures drop battery voltage, causing weak spark and misfires. For Peugeot/Citroen, the 'Injector Learning' procedure requires a minimum intake air temperature (>30°C), making the software fix impossible outdoors in winter without a heat gun.
• High Humidity / Salty Regions: Moisture and road salt corrode the older metal-cased Ford DPFE sensors, triggering the 'EGR Calibration High' fault. Salt also degrades ignition coil grounds, causing intermittent misfires.
• High Altitude: Thinner air exacerbates minor vacuum leaks or lazy oxygen sensors, pushing the air-fuel mixture lean and triggering misfire codes.

How to Talk to a Mechanic About This Code

Say this: "I have a P1304 code on my [Make and Model]. I know this is a manufacturer-specific code meaning [misfire/EGR calibration/injector learning]. For a misfire, please start with basic spark and fuel checks. For a Ford EGR code, please test the DPFE sensor voltage before replacing parts. For a Peugeot/Citroen, do you have the brand-specific scan tool to perform an injector calibration?"

This proves you understand the specific nature of the code. It guides the mechanic toward a logical diagnostic process, preventing them from replacing unnecessary parts.

Avoid saying:

• 'My check engine light is on, can you look at it?' (Invites a wide-ranging, expensive diagnostic).
• 'Just fix whatever's wrong.' (Gives the shop a blank check).
• 'The internet said it's the coil.' (Demanding parts replacement without diagnosis wastes money if you are wrong).

Questions to ask before authorizing the repair:

• Can you show me the diagnostic reading that confirms the failure (e.g., the high DPFE voltage or the failed compression test)?
• What is the total estimated cost, including parts, labor, and taxes?
• What is the warranty on this specific repair, covering both the part and the labor?
• If this repair doesn't fix the P1304 code, what is the next diagnostic step, and will I be charged again for diagnosis?

Where to Take It: Dealer vs Independent vs Chain

• Dealer:
  Best for: Vehicles under powertrain or emissions warranty., The 'Injector Learning' P1304 variant on Peugeot/Citroen/Fiat requiring proprietary software., Complex electrical issues independent shops cannot diagnose.
  Downsides: Labor rates are 40% higher than independent shops., Dealers replace expensive assemblies instead of smaller failed components. (Typical cost: +40% vs. baseline)
• Independent Shop: Best fit for misfire and EGR variants. For the 'Injector Learning' version, confirm they possess the specific diagnostic tool before booking.
  Best for: Out-of-warranty vehicles., The 'Cylinder 4 Misfire' variant (Honda, Saab)., The 'EGR Calibration' variant on Fords.
  Downsides: Quality varies widely; verify ASE certifications., May lack the brand-specific tools needed for Peugeot injector calibration. (Typical cost: +0% vs. baseline)
• Chain Shop: Acceptable for a straightforward coil swap. AVOID for initial diagnosis of P1304 due to its varied meanings.
  Best for: Simple parts replacement (ignition coil or DPFE sensor) only after a proper diagnosis is complete.
  Downsides: Technician skill is inconsistent., High pressure to upsell unnecessary fluid flushes., Incapable of in-depth diagnostic work on manufacturer-specific codes. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 40-50% of the car's private-party value, sell or trade in the vehicle instead of repairing it.

• Car worth $4000, fix is $2200: Walk away. The repair cost is over 50% of the car's value. Put that money toward a replacement vehicle.
• Car worth $5000, fix is $450: Fix it. A $450 repair for a new DI cassette on a Saab 9-5 is well below the threshold and restores the car.
• Car worth $2500, fix is $1500: Walk away. A $1,500 repair on a $2,500 car is a poor investment; other age-related failures are imminent.

What Scan Tool You Need for This Code

Minimum: A scanner that reads manufacturer-specific codes and views live data (DPFE sensor voltage, engine RPM, load).

A $20 code reader only shows the code number, not the manufacturer-specific definition or live sensor data. This forces you to guess and waste money on unnecessary parts.

Budget: BlueDriver Pro (~$100) — Connects via Bluetooth. Reads manufacturer-specific definitions for P1304, graphs live data (essential for DPFE voltage), and reads freeze-frame data.

Mid-range: Foxwell NT510 Elite (~$200) — Includes one free manufacturer software package. Offers bidirectional controls needed to run specific active tests, making it viable for the Peugeot/Citroen 'Injector Learning' variant.

Professional: Autel MaxiCOM MK808S (~$450) — A full diagnostic tablet with powerful bidirectional controls. Performs advanced injector coding for Peugeot/Citroen and active tests to command the EGR valve or fuel pump.

Rent vs buy: For a one-time diagnosis, use AutoZone's 'Loan-A-Tool' program. You pay a deposit equal to the tool's cost, fully refunded upon return. If you repair cars yearly, buy the BlueDriver.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the diagnostic trouble code.
2. Perform a complete drive cycle to allow the vehicle's readiness monitors to run.
3. Rescan for any pending or active codes to confirm the fix.

Drive cycle (~30 minutes): A general drive cycle includes a cold start (engine off for 8+ hours), 2-3 minutes of idling, 10-15 minutes of mixed city driving, followed by 10 minutes of steady highway driving at 55-60 mph.

Readiness monitors affected: Misfire Monitor, Catalyst Monitor, EGR System Monitor, Oxygen (O2) Sensor Monitor

Before emissions retest: drive at least 50-100 miles to fully set monitors.

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors to 'Not Ready', causing an automatic emissions failure.
• The code returns immediately if the underlying mechanical or electrical fault is not repaired.
• Failing to complete a full drive cycle leaves the Catalyst and EGR monitors incomplete.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An active P1304 is an automatic failure. After repair, all readiness monitors (except EVAP) must be 'Ready' to pass the smog check. Drive 50-100 miles to reset monitors.
• New York: The NYS DMV inspection includes an OBD-II scan. An illuminated Check Engine Light for P1304 results in immediate failure.
• Texas: In the 17 counties requiring emissions testing, an active P1304 causes failure. After clearing, you can have at most one monitor 'Not Ready' (two for 1996-2000 models) to pass.

Most Commonly Affected Vehicles

• Saab 9-3, 9-5 (1998-2010) — P1304 guarantees a failing Direct Ignition (DI) cassette. The T7 engine requires a black cassette (Part #55559955). Cheap aftermarket cassettes cause repeat failures.
• Ford F-150, Ranger, Explorer, Mustang (1996-2008) — Triggers the 'EGR Calibration High' variant. Caused by a faulty DPFE sensor (especially pre-2004 metal-cased units) or clogged EGR passages.
• Acura / Honda TL, CL, MDX, Accord (1999-2013) — Indicates 'Cylinder 4 Misfire'. TSB 13-024 warns that persistent misfires on 2013 models stem from out-of-round cylinder bores requiring engine replacement.
• Peugeot / Citroen Boxer, Relay (2006-2014) — Indicates 'Injector Learning Not Performed' on 2.2 HDI engines. Sets after replacing injectors without calibrating them to the ECU using DiagBox software.
• BMW 3-series (E46), 5-series (E39) (1998-2006) — Indicates a cylinder 4 misfire. Frequently caused by a leaking valve cover gasket pooling oil into the spark plug well and destroying the ignition coil boot.
• Hyundai Elantra, Tiburon, Sonata (2001-2012) — Defined as a 'Phase Sensor' fault (Camshaft Position Sensor). Diagnosis must focus on the sensor circuit, not cylinder 4 ignition components.
• Mazda 3, 6, CX-7 (2010-2017) — Indicates Cylinder 4 Misfire. TSB #01-014-16-3208A addresses failing YUMEBISHI-brand ignition coils. TSB #01-004-17-3317a points to PCM failure on 2014-2015 models.
• Jaguar / Land Rover X-Type, S-Type, Discovery, Range Rover (2002-2012) — Indicates Cylinder 4 Misfire. On the 5.0L V8, negative fuel trims alongside misfire codes point to leaking fuel injectors.

Manufacturer-Specific Notes

• Ford / Lincoln / Mercury: P1304 means 'EGR Calibration High'. It is almost exclusively caused by a failed DPFE sensor sending 5V to the PCM. Replace the old metal sensor with the updated plastic Motorcraft version.
• Saab: P1304 means 'Cylinder 4 Misfire'. The Trionic 7 system uses spark plugs as combustion sensors. Installing non-NGK spark plugs disrupts this feedback loop and destroys the DI cassette.
• Peugeot / Citroen / Fiat: P1304 means 'Injector Programming'. It is a software lock triggered when new injectors are installed without ECU calibration. You cannot clear the code without completing the learning procedure via a specialized scan tool.
• Hyundai: P1304 means 'Phase Sensor' fault. Do not replace ignition coils; test the Camshaft Position Sensor and its wiring harness.
• Mazda: P1304 means 'Cylinder 4 Misfire'. Mazda issued TSBs for faulty YUMEBISHI ignition coils and failing PCMs causing this exact code. Check warranty coverage before paying out of pocket.

Real Owner Stories

2004 Ford F-150 with 5.4L V8 at 145K miles

Check Engine Light illuminated with codes P1304 and P0401. The truck had a rough idle and hesitated under light acceleration.

What they tried:

1. Replaced the EGR valve, but the codes returned within a day.
2. Checked the vacuum lines to the EGR and DPFE sensor; found them intact.
3. Tested the DPFE sensor voltage at idle and found it stuck at 4.5V, far above the 0.5-1.0V specification.

Outcome: Replaced the original metal-cased DPFE sensor with an updated Motorcraft plastic sensor (DPFE-15) for $60. The idle smoothed out immediately, and the codes never returned.

Lesson: For Ford P1304/P0401, test the DPFE sensor voltage before replacing the expensive EGR valve. A sensor stuck near 5V is a guaranteed failure.

2003 Saab 9-5 Aero at 110K miles

Car ran violently rough with a flashing Check Engine Light. Codes pulled were P1304 and P1312.

What they tried:

1. Suspected the Direct Ignition (DI) cassette based on Saab's known failure points.
2. Swapped in a known-good DI cassette from a donor car. The engine ran smoothly.
3. Inspected the spark plugs and found incorrect, heavily worn aftermarket plugs.

Outcome: Installed a genuine Saab DI cassette (part #55559955) for $350 and a set of NGK BCPR7ES-11 spark plugs. The car ran perfectly.

Lesson: On a Saab with multiple misfire codes, the DI cassette is the culprit. Using cheap aftermarket cassettes or non-NGK spark plugs guarantees premature failure.

2008 Honda Accord at 120K miles

Persistent P1304 (Cylinder 4 Misfire) code with a noticeable vibration at idle.

What they tried:

1. Replaced the spark plug in cylinder 4; no change.
2. Swapped the ignition coil from cylinder 4 to cylinder 3. The P1304 returned; the misfire did not follow the coil.
3. Tested the fuel injector on cylinder 4 with a multimeter and found infinite resistance (open circuit).

Outcome: Replaced the cylinder 4 fuel injector for $120. The P1304 code cleared and the idle smoothed out.

Lesson: If a misfire code doesn't move during a coil swap test, test the fuel injector's electrical resistance before assuming internal engine damage.

2010 Peugeot Boxer 2.2 HDI

Van refused to start and showed P1304 'Injector Learning Not Performed' after a garage replaced a fuel injector.

What they tried:

1. The garage attempted to clear the code with a generic OBD-II scanner, which failed.
2. Took the van to a specialist with DiagBox software.
3. The specialist found the learning procedure failed because the intake air temperature was below the 30°C prerequisite.

Outcome: Warmed the intake air temperature sensor with a heat gun until it read above 30°C. Successfully ran the injector quantity calibration procedure. The van started immediately.

Lesson: For Peugeot/Citroen/Fiat, P1304 is a software lock. You must use a brand-specific scan tool to run the calibration, and you must meet all temperature prerequisites for it to succeed.

How to Prevent This Code From Triggering

• Use Top-Tier Certified Gasoline (Every fill-up) — Top Tier gasolines contain high concentrations of detergents that prevent carbon deposits on fuel injectors and intake valves, eliminating lean misfires.
• Follow Spark Plug Replacement Intervals (Per manufacturer schedule (typically 60k-100k miles)) — Worn spark plugs force the ignition coil to produce higher voltage to jump the widened gap. This extra strain overheats and destroys coils prematurely.
• Perform Periodic Highway Driving (At least 30 minutes weekly) — Sustained highway speeds allow the engine to burn off carbon deposits from the EGR valve, intake, and catalytic converter.
• Clean Ignition Coil Connections (During spark plug changes) — Inspect coil boots for oil contamination from leaking valve cover gaskets. Clean connections and apply dielectric grease to seal out moisture and ensure strong spark delivery.

Frequently Asked Questions

Can I fix a P1304 code myself?

Yes, if the code indicates a misfire, replacing a spark plug or ignition coil requires only basic tools. Diagnosing a Ford EGR issue requires a multimeter but is highly DIY-friendly. Software-related P1304 codes on European diesels require professional scan tools.

What happens if I ignore a P1304 code?

Ignoring a misfire dumps unburnt fuel into the exhaust, destroying your catalytic converter and causing a $2,500 repair. Ignoring an EGR fault causes engine pinging and automatic emissions test failure.

Why is my Check Engine Light flashing with a P1304 code?

A flashing light warns of a severe, catalyst-damaging misfire dumping raw fuel into the exhaust. Reduce speed, avoid heavy acceleration, and pull over safely. Driving even a few miles will melt the catalytic converter.

I replaced the coil and spark plug, but P1304 came back. What now?

Test the fuel injector electrically with a multimeter and check for a pulse signal using a noid light. Inspect for vacuum leaks and check the coil connector wiring. If those pass, perform a compression test to diagnose internal mechanical failures.

My car has the 'Injector Learning' version of P1304. Can I fix it by just clearing the code?

No. The engine computer (ECU) has locked out because it lacks calibration data for the injector. You must use an advanced diagnostic scan tool to complete the specific 'injector learning' procedure. Generic code readers cannot bypass this.

How do I find cylinder 4?

On a 4-cylinder engine, cylinder 4 is the last cylinder at the back of the engine, furthest from the drive belts. On a V6 or V8, numbering depends on the manufacturer, so consult a specific engine diagram. For example, on Ford V8s, it is the rear-most cylinder on the passenger side.

Can a bad battery or alternator cause a P1304 code?

Yes. A weak battery or failing alternator provides unstable voltage to the electrical system. This degrades ignition coil and PCM performance, triggering weak spark and misfire codes.

Key Takeaways

• P1304 is a manufacturer-specific code; verify your vehicle's exact definition before buying parts, as it means 'Cylinder 4 Misfire' on Hondas but 'EGR Calibration High' on Fords.
• If your Check Engine Light is flashing, pull over immediately; driving with a severe misfire destroys a $2,000 catalytic converter in less than 10 miles.
• For a misfire definition, swap the cylinder 4 ignition coil with cylinder 1; if the code changes to P0301, replace the $60 faulty coil.
• For Ford vehicles, test the DPFE sensor voltage with a multimeter; a reading stuck near 5V confirms a failed sensor requiring a $50 replacement.
• On Peugeot, Citroen, and Fiat diesels, P1304 requires a specialized diagnostic tool to perform a mandatory software 'injector learning' procedure.