OBD-II Code P0304: Cylinder 4 Misfire Detected

What Does P0304 Mean?

Code P0304 means the Powertrain Control Module (PCM) detected cylinder 4 is not firing correctly. A cylinder requires sufficient compression, a precise air/fuel mixture, and a well-timed spark to fire. When one of these fails in cylinder 4, the engine loses power, runs rough, and increases emissions.

Technical definition: The SAE/OBD-II definition is "Cylinder 4 Misfire Detected." 🎬 Watch: This breakdown helps you find the real cause fast. The PCM monitors crankshaft rotational speed and registers deceleration events—often just a 2% speed variation—after cylinder 4's scheduled power stroke. If the variance exceeds 10%, the PCM flashes the check engine light, indicating a catalyst-damaging misfire.

Can I Drive With P0304?

Yes, But With Caution. Do not drive with a flashing check engine light. A flashing light indicates a severe misfire dumping unburned fuel into the exhaust, destroying your catalytic converter in minutes. If the light is solid, drive cautiously directly to a repair shop, avoiding heavy acceleration. Continuing to drive turns a $150 ignition fix into a $2,500+ catalytic converter replacement.

Common Causes

• Faulty Ignition System Component (Spark Plug, Ignition Coil, or Wire) (Very Common) — This is the most frequent cause. Spark plugs wear out, ignition coils fail from heat and vibration, and spark plug wires degrade, preventing a strong spark in cylinder 4.
• Faulty or Clogged Fuel Injector (Common) — The cylinder 4 fuel injector clogs with carbon deposits or fails electronically, preventing the correct fuel volume from spraying into the cylinder and causing a lean misfire.
• Excessive Carbon Buildup on Intake Valves (Common) — On Gasoline Direct Injection (GDI) engines, fuel sprays directly into the cylinder, bypassing the intake valves. Oil vapor bakes onto the valves, restricting airflow into cylinder 4 and causing cold-start misfires.
• Vacuum Leak (Less Common) — A leaking vacuum hose or failed intake manifold gasket near cylinder 4 allows unmetered air into the engine, disrupting the air-to-fuel ratio and causing a lean misfire.
• Active/Variable Fuel Management System Fault (Less Common) — On engines with GM's Active Fuel Management (AFM) or Honda's Variable Cylinder Management (VCM), cylinder 4 deactivates to save fuel. The specialized lifters stick or fail, causing a persistent misfire.
• Internal Engine Mechanical Problems (Rare) — A leaking head gasket, bent valve, worn piston rings, broken valve spring, or damaged camshaft lobe causes compression loss in cylinder 4. Diagnosed via compression and leak-down tests.
🎬 Watch a pro mechanic diagnose a cylinder four misfire.
• Faulty Engine Control Module (ECM) or Wiring (Rare) — A damaged wiring harness (corroded pins, short to ground) or a faulty driver circuit within the ECM fails to send correct signals to the cylinder 4 ignition coil or fuel injector.

Symptoms

• Flashing or Solid Check Engine Light — A flashing light indicates a severe, catalyst-damaging misfire. A solid light indicates a persistent issue requiring prompt attention.
• Rough Engine Idle and Vibration — The engine shakes or vibrates noticeably at idle or low RPMs, often feeling like it will stall.
• Engine Hesitation or Stumbling — The car struggles to accelerate smoothly, jerking or stumbling under load, especially when climbing hills.
• Reduced Fuel Economy — Inefficient combustion in cylinder 4 drops fuel economy by 5-15%.
• Smell of Unburned Fuel from Exhaust — A failing cylinder sends raw fuel into the exhaust system, creating a distinct gasoline smell during a severe misfire.

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 — Parts: $40-$90 (per coil), Labor: $75-$150, ~0.8 hr book time (DIY)
• Replace Spark Plugs — Parts: $10-$40 (per plug), Labor: $100-$250 (for a set), ~1.5 hr book time (DIY)
• Replace Fuel Injector — Parts: $100-$500 (per injector), Labor: $150-$400, ~3 hr book time (Intermediate)
• Intake Valve Carbon Cleaning (Walnut Blasting) — Parts: $50-$100 (materials), Labor: $400-$900 (for professional service), ~3.5 hr book time (Professional)
• Intake Manifold Gasket Replacement — Parts: $20-$70, Labor: $200-$600, ~4 hr book time (Professional)

DIY vs Professional

• Replace Spark Plugs / Ignition Coil — Beginner:
  Tools: Basic socket set, spark plug socket, torque wrench, dielectric grease.
• Replace Fuel Injector (Port Injection) — Beginner:
  Tools: Socket set, fuel line disconnect tools, new O-rings.
• Replace Fuel Injector (Direct Injection) — Beginner:
  Tools: Specialized tools for high-pressure fuel lines, new one-time-use lines/seals, advanced service manual procedures.
• Intake Manifold Gasket / Carbon Cleaning — Beginner:
  Tools: Extensive tool set, torque wrench, specific scrapers, walnut blasting equipment.

Used vs. New Parts: Buying Guide

When a used part is worth it: Always buy new spark plugs and ignition coils. Use a used ignition coil only as a temporary emergency fix to reach a repair shop.

Donor-vehicle mileage cap: roughly under 40000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Visually inspect used coils for cracks, corrosion on electrical pins, or heat damage on the boot.
• Match the part number exactly; similar-looking coils have different internal resistance.
• Avoid parts from vehicles scrapped due to engine failure or flood damage.

Decision logic:

• If The part is a spark plug → Always buy new. They are inexpensive wear items.
• If The part is an ignition coil → Buy a new OEM or high-quality aftermarket coil. The risk of early failure and repeat labor costs outweighs the savings of a used part.
• If The budget is extremely tight and the vehicle is stranded → A used coil functions temporarily but carries a significantly shorter lifespan.

Warranty tradeoff: Used electrical parts typically have a 30-90 day functional warranty excluding labor. New aftermarket parts carry 1-year to limited lifetime warranties; new OEM parts offer 1-2 year warranties.

Worst-case if a used part fails: $150-300 if a used ignition coil fails shortly after installation, requiring repeat labor and a new part.

What Happens If You Wait — Timeline

1. 0-2 weeks: Solid check engine light appears. Misfire felt as a slight rough idle, often when cold. (MPG impact: 1-5%% · Added cost: $0-25 in wasted fuel)
2. 2 weeks - 2 months: Engine vibration at idle becomes noticeable. Hesitation during acceleration is frequent. Unburned fuel intermittently super-heats the catalytic converter. (MPG impact: 5-10%% · Added cost: $50-150 in wasted fuel and early catalyst stress)
3. 2-6 months: Check engine light flashes during heavy acceleration. The catalytic converter's internal substrate cracks or melts. The car smells like rotten eggs. (MPG impact: 10-20%% · Added cost: $800 - $2,500 (catalytic converter replacement required))
4. 6+ months: Catastrophic catalyst failure. The melted honeycomb creates a major exhaust blockage. Engine power is severely limited, and the engine stalls. (MPG impact: 20-50%+% · Added cost: $1,500 - $4,000+ (includes catalytic converter, O2 sensors, and engine diagnostics))

Cost of Not Fixing It

• Immediate (0-1 Month): Noticeable drop in fuel economy (5-15%), rough idle, and poor acceleration. Failed emissions test is guaranteed. (Added cost: Negligible, besides increased fuel cost.)
• Short-Term (1-3 Months): Unburned fuel from cylinder 4 overheats and damages the catalytic converter. Oxygen sensors foul. (Added cost: $200-$500 for oxygen sensor replacement.)
• Long-Term (3+ Months): Catastrophic failure of the catalytic converter. The internal honeycomb structure melts, causing a major exhaust blockage and severe engine damage. (Added cost: $1,200-$3,000+ for catalytic converter replacement.)

Diagnosis Steps

1. Read the Codes and Freeze Frame Data
   Use an OBD-II scanner to confirm P0304. Check for context codes (e.g., P0300, P0171, P0354). Review freeze frame data to determine if the misfire happens when cold, hot, or under load.
   Tools: OBD-II Scanner (Beginner)
2. Swap Ignition Components (The 'Money-Saver' Test)
   Isolate the ignition problem by swapping the cylinder 4 ignition coil with cylinder 1. Clear codes and drive. If the code changes to P0301, the ignition coil is faulty. Repeat with the spark plug if the coil is good.
   Tools: Socket set, Spark plug socket (Intermediate)
3. Visually Inspect Cylinder 4 Components
   Locate cylinder 4 using a service manual. Inspect the spark plug wire, ignition coil, and fuel injector wiring for damage, corrosion, loose connections, or fluid contamination.
   Tools: Flashlight, Service Manual (Beginner)
4. Test the Fuel Injector
   With the engine running, listen to the cylinder 4 fuel injector with a mechanic's stethoscope. It should make a consistent, rhythmic clicking. Connect a 'noid light' to the electrical connector to verify the ECM pulse signal.
   Tools: Mechanic's stethoscope, Noid light set (Intermediate)
5. Check for Vacuum Leaks
   With the engine running, spray carburetor cleaner or use a smoke machine around the intake manifold gasket near cylinder 4. If the idle changes or smoke draws in, you have a vacuum leak.
   Tools: Carburetor cleaner or Smoke machine (Intermediate)
6. Perform an Engine Compression Test
   Thread a compression tester into the cylinder 4 spark plug hole and crank the engine. A reading below 130 PSI or a variance >15% between cylinders indicates internal engine damage (bad rings, valves, or head gasket).
   Tools: Compression tester kit (Advanced)
7. Pro Tip: Test Ignition Coil Primary/Secondary Circuit
   Disconnect the coil and use a multimeter set to ohms (Ω). Measure primary windings (0.4-2.0 ohms) and secondary windings (6,000-10,000 ohms). Out-of-spec readings confirm a bad coil. Compare to service manual specs.
   Tools: Multimeter, Service Manual (Advanced)
8. Pro Tip: Test Fuel Injector Resistance
   Disconnect the injector and measure resistance across the terminals with a multimeter. Standard port injectors read 10-15 ohms. All injectors should read within 0.5 ohms of each other. A 20% variance indicates failure.
   Tools: Multimeter, Service Manual (Advanced)
9. Pro Tip: Check Fuel Pressure
   Connect a fuel pressure gauge to the fuel rail. Port-injected engines should idle at 40-65 PSI. A weak pump or clogged filter causes lean misfires across all cylinders.
   Tools: Fuel Pressure Gauge, Service Manual (Advanced)
10. Pro Tip: Perform a Cylinder Leak-Down Test
    Feed pressurized air into cylinder 4 at TDC. A leak >20% or >5% difference between cylinders is a concern. Listen for escaping air: oil cap (piston rings), throttle body (intake valve), or tailpipe (exhaust valve).
    Tools: Cylinder Leak-Down Tester, Air Compressor (Professional)
11. Pro Tip: Analyze Fuel Injector Waveform with an Oscilloscope
    Connect an oscilloscope to the injector command wire. A good waveform shows a sharp voltage drop when opening, a pintle hump, and a large inductive kick when closing. Malformed patterns indicate a failing injector or ECM driver.
    Tools: Automotive Oscilloscope (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-205°F (Engine is at full operating temperature.)
• RPM: 1500-3000 RPM (Misfire is often detected under light to moderate load, such as steady cruising.)
• Engine Load: 25-60% (Indicates the engine is working, but not under heavy acceleration.)
• Vehicle Speed: 40-65 mph (Corresponds to city or highway cruising speeds where misfires are stable enough to be reliably counted by the PCM.)

Related Codes

• P0300 — Indicates 'Random/Multiple Cylinder Misfire.' If P0300 appears with P0304, the cause affects the whole engine. Suspect a major vacuum leak, low fuel pressure, or a bad Mass Airflow (MAF) sensor.
• P0171 or P0174 — Indicates a 'System Too Lean' condition. A vacuum leak near the intake manifold runner for cylinder 4 causes both P0304 and a lean code. Diagnose with a smoke test.
• P0420 or P0430 — Indicates 'Catalyst System Efficiency Below Threshold.' These are consequences of a P0304 misfire. Unburned fuel destroys the catalytic converter. Fix the misfire first.
• P0354 — Indicates 'Ignition Coil D Primary/Secondary Circuit Malfunction.' The PCM detected an electrical fault with the cylinder 4 ignition coil or circuit. The coil or wiring is the culprit.

Climate & Environmental Factors

• Cold Weather: Cold temperatures thicken engine oil, strain batteries, and worsen issues with weak ignition components or leaky fuel injectors, making cold-start misfires more likely. On GDI engines, short cold trips lead to fuel dilution of the oil.
• High Humidity: Moisture exposes weaknesses in the ignition system. High voltage arcs to ground through cracks in ignition coil housings or old spark plug wires instead of firing the plug, causing damp-weather misfires.
• High Altitude: Less dense air naturally creates a richer air-fuel mixture. While modern ECUs compensate, altitude exacerbates existing issues with fouled spark plugs or lazy oxygen sensors.

How to Talk to a Mechanic About This Code

Say this: "I have a P0304 code indicating a cylinder 4 misfire. Please start the diagnostic by swapping the cylinder 4 ignition coil to another cylinder to see if the code follows it. Call me with the result of that specific test before proceeding."

This directs the mechanic to perform the most logical and cost-effective first step, preventing expensive diagnostics or unnecessary parts replacement.

Avoid saying:

• 'My car is shaking, just fix it.'
• 'My check engine light is on, can you look at it?'
• 'Just replace the spark plugs and coils.' (The problem could be a fuel injector or mechanical issue)

Questions to ask before authorizing the repair:

• Did the misfire code move to the other cylinder when you swapped the coil?
• If the coil swap didn't move the code, what were the results of the spark plug swap?
• If it's not the coil or plug, what is the next diagnostic step you recommend, and what will it cost?
• Will you provide a written estimate with a breakdown of parts and labor before you proceed?
• What is your warranty on this repair, for both parts and labor?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended only if the vehicle is under warranty or has a known, complex TSB-related issue.
  Best for: Vehicles under powertrain or emissions warranty., Known, complex manufacturer-specific quirks (e.g., GM's AFM lifter issues, Honda's fuel dilution TSBs)., Hard-to-diagnose issues after an independent shop has failed.
  Downsides: Highest labor rate, typically 1.5-2x more than an independent shop., May recommend replacing parts in sets (e.g., all 4 coils) when only one has failed. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most situations. A P0304 is a standard job for any competent independent technician.
  Best for: Most out-of-warranty vehicles., Diagnosing and fixing common codes like P0304., Building a long-term relationship with a trusted mechanic.
  Downsides: Quality and expertise vary widely. Vet shops based on reviews and ASE certifications., May lack expensive, specialized tools for the very latest models. (Typical cost: +0% vs. baseline)
• Chain Shop: Use with caution. Acceptable if you have already diagnosed a bad ignition coil and just need it replaced. AVOID for initial diagnosis.
  Best for: Simple, clear-cut repairs where the faulty part is already known., Tires, batteries, and oil changes.
  Downsides: Technician skill and diagnostic capability vary dramatically., Business model creates high pressure for upselling unnecessary services. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the P0304 repair estimate exceeds 40-50% of the car's Kelley Blue Book private-party value, sell or trade in the vehicle.

• Car worth $5000, fix is $250: Fix it. A coil and plug replacement is a minor cost relative to the car's value.
• Car worth $8000, fix is $3500: Walk away. If the P0304 is caused by a major mechanical failure like a bad AFM lifter, the repair cost is nearly half the vehicle's value.
• Car worth $2500, fix is $800: Borderline. The repair for a bad fuel injector or carbon cleaning is a significant percentage of the car's value. Get a second opinion.

What Scan Tool You Need for This Code

Minimum: A scanner that reads codes, displays live data, and accesses Mode $06 misfire counters.

A basic $20 code reader won't show live misfire counts per cylinder (Mode $06), which is critical to confirm the misfire in real-time and verify the fix.

Budget: BlueDriver Pro (~$100) — Connects to your smartphone to provide Freeze Frame data, live data graphing, and Mode $06 raw misfire counts.

Mid-range: Foxwell NT510 Elite / Autel MaxiCOM MK808 (~$180-500) — Adds bidirectional controls. Allows you to perform an 'active test' to command the cylinder 4 fuel injector on and off, isolating a faulty injector.

Professional: Autel MaxiSys MS906 / Launch X431 Series (~$1200+) — Provides full OEM-level diagnostics and interfaces with an oscilloscope for detailed waveform analysis of ignition and injector circuits.

Rent vs buy: Auto parts stores read codes for free, but their scans won't show Mode $06 data. Owning a tool like the BlueDriver ($100) allows you to perform the 'swap test' and verify the fix yourself, saving its cost on a single repair.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the P0304 code.
2. Perform a complete OBD-II drive cycle to allow readiness monitors to run.
3. Rescan the vehicle to confirm the code has not returned and monitors are set.

Drive cycle (~30 minutes): A generic drive cycle includes: 1) Cold start (engine temp below 122°F). 2) Idle for 2-3 minutes. 3) Drive at a steady speed of 55 mph for 5-10 minutes. 4) Perform 5-10 slow decelerations to 20 mph without braking. 5) Engage in 15-20 minutes of mixed city/highway driving with several idle periods.

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

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

Watch out for:

• Clearing the code with a scanner does not fix the underlying problem; the code will return.
• Disconnecting the battery resets all readiness monitors to 'Not Ready', causing an automatic emissions test failure.
• Failing to complete the full drive cycle leaves monitors 'Not Ready', preventing emissions test passage.

Will This Fail Emissions / State Inspection?

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

• California: An active P0304 code is an automatic failure. CARB requires most or all readiness monitors to be 'Ready' post-repair. Driving 100+ miles of varied conditions is required before retesting.
• New York: An illuminated Check Engine Light fails the NYS inspection. For 2001+ vehicles, only one non-continuous readiness monitor can be 'Not Ready'.
• Texas: In the 17 emissions-testing counties, an illuminated Check Engine Light is an automatic failure. For 2001+ vehicles, only one non-continuous monitor can be 'Not Ready'.

Most Commonly Affected Vehicles

• Ford F-150 / Expedition (5.4L 3V Triton) (2004-2010) — The cylinder #4 ignition coil sits under a heater hose that leaks coolant directly onto it, shorting the coil. Fix the coolant leak when replacing the coil.
• Chevrolet / GMC Silverado / Sierra 1500 (5.3L V8) (2014-2023) — These engines feature Active Fuel Management (AFM). P0304 is often linked to a stuck or failed AFM lifter on cylinder 4, indicated by a persistent ticking noise.
• Honda CR-V / Civic (1.5L Turbo) (2016-2021) — Known for fuel dilution of the engine oil in cold climates with short trips, causing misfires. Honda issued TSBs and warranty extensions to address this.
• BMW Various (3-Series, 5-Series, X3, X5) (2006-2017) — Highly sensitive to ignition component quality. P0304 is commonly caused by a failed ignition coil. Replace all plugs and coils simultaneously to prevent future misfires.
• Hyundai / Kia Forte / Elantra / Soul / Optima (2012-2020) — Ignition coil failure is extremely common. TSBs recommend software updates for ECU misfire detection logic or replacing all four coils to resolve persistent issues.
• Volkswagen / Audi Golf / Jetta / GTI / A4 (TSI/FSI Engines) (2009-2021) — If new plugs and coils do not resolve the misfire, heavy carbon buildup on the intake valves is the culprit, requiring professional walnut blasting at 60,000+ miles.
• Subaru Outback / Legacy / Impreza / WRX (2005-2014) — The plastic connector for the ignition coil becomes brittle and fails, leading to a poor connection. Inspect the wiring harness connector for cracks.
• Jeep Grand Cherokee (4.7L V8) (1999-2010) — Prone to fuel injector failures triggering P0304. Swapping the injector to another cylinder confirms the failure.

Manufacturer-Specific Notes

• Ford: On 5.4L 3V engines, the #4 ignition coil fails due to coolant leaks from an overhead heater hose. On 2019+ Rangers, TSB 23-2011 addresses water intrusion into the #4 spark plug well.
• Chevrolet / GMC: On 5.3L and 6.2L V8 engines with AFM, cylinder 4 deactivates. Stuck or collapsed AFM lifters are a widely documented cause of P0304 requiring internal engine repair.
• Honda: 1.5L Turbo engines (2016-2018 Civics, 2017-2018 CR-Vs) suffer from fuel diluting the engine oil, leading to misfires. Honda extended warranties for affected components.
• Kia / Hyundai: Prone to ignition coil failures. TSBs (e.g., TSBENG181_R1) address misfires by recommending ECU software updates or replacing all ignition coils as a set.

Real Owner Stories

2001 Jeep Grand Cherokee 4.7L V8 - Intermittent Misfire

Vehicle showed a P0304 code with intermittent misfires. A previous diagnosis confirmed a bad fuel injector by swapping it and seeing the code follow the injector.

What they tried:

1. Injector swap to confirm the faulty component.

Outcome: The owner confirmed the need to replace the specific fuel injector but had to delay the repair, raising concerns about driving safety.

Lesson: If an injector is confirmed bad via a swap test, replace it promptly. Driving with a sticking injector risks washing fuel past piston rings and damaging the catalytic converter.

2006 Ford Five Hundred - Persistent P0304 After Initial Fix

The car experienced jerking and shaking while idling and accelerating, with a P0304 code. A mechanic replaced the ignition coil and spark plug.

What they tried:

1. Replaced ignition coil and spark plug at a mechanic.
2. Returned to the mechanic when the light came back; they replaced the coil again.

Outcome: The check engine light returned with the same P0304 code a third time. The owner spent over $1000 without a fix.

Lesson: If a P0304 code returns after replacing the plug and coil, the issue is deeper. Suspect a faulty fuel injector, wiring issue, or low compression. Insist on further diagnostics.

2002 Ford Escort 2.0L SOHC - Intermittent Cold Start Misfire

An intermittent P0304 code appeared on startup, lasting less than 30 seconds.

What they tried:

1. Changed spark plugs.
2. Swapped spark plug wires between cylinders 3 and 4.
3. Swapped the coil pack with a known good one.

Outcome: The P0304 code persisted. Forum members warned of a notorious issue on this specific engine where the #4 exhaust valve seat drops, causing these exact symptoms.

Lesson: For intermittent, temperature-sensitive misfires, research forums for known engine-specific mechanical failures. A simple ignition problem could be a sign of a documented valve seat failure.

1999 Dodge Stratus 2.4L - Rough Idle and Flashing CEL on Load

Vehicle had a rough idle and flashed the check engine light when accelerating or going uphill. The code was P0304.

What they tried:

1. Changed the coil pack.

Outcome: The rough idle and misfire symptoms continued. The next steps were investigating the fuel injector or checking for mechanical issues.

Lesson: A flashing check engine light under load is a critical sign of a severe misfire. If replacing a coil pack fails, the problem is likely fuel-related or mechanical and requires immediate diagnosis.

How to Prevent This Code From Triggering

• Use Top Tier certified gasoline (Every fill-up) — Top Tier gas contains detergents that prevent carbon deposits on fuel injectors and intake valves, ensuring a proper fuel spray pattern.
• Replace spark plugs at the recommended interval (Every 60,000-100,000 miles) — As the electrode erodes, the voltage required to create a spark increases, straining the ignition coil. Worn plugs cause misfires and premature coil failure.
• Use a PEA-based fuel system cleaner (Every 10,000-15,000 miles) — Polyetheramine (PEA) cleans soft carbon deposits from the fuel system, preventing injector clogging.
• Perform regular visual inspections of ignition components (During every oil change) — Catching cracked ignition coil housings, corroded connectors, or oil/coolant leaks early prevents future P0304 codes.
• Change engine oil on schedule with quality oil (Per manufacturer's recommendation) — Clean oil is critical for VVT solenoids and hydraulic lifters (in AFM/VCM systems). Dirty oil causes these components to malfunction, leading to misfires.

Frequently Asked Questions

Can I drive my car with a P0304 code?

You can drive short distances with a solid check engine light, but avoid heavy acceleration. If the light flashes, stop driving immediately. A flashing light means raw fuel is destroying your catalytic converter, turning a minor repair into a $2,000+ bill.

What is the most common cause of a P0304 code?

The most frequent cause is a failed ignition system component on cylinder 4. This typically means a worn spark plug or a burnt-out ignition coil.

How much does it cost to fix a P0304 code?

Costs range from $110 for a DIY spark plug replacement to over $3,500 for internal engine repairs. The most common fix—replacing an ignition coil at a shop—averages $150 to $240. Always diagnose the root cause before buying parts.

I just replaced my spark plugs and now I have a P0304 code. What happened?

The new spark plug for cylinder 4 is likely defective, improperly gapped, or cracked during installation. Alternatively, the ignition coil connector failed to seat properly. Re-check your work and torque specifications on cylinder 4.

My check engine light is flashing. What does that mean?

A flashing check engine light signals a severe, catalyst-damaging misfire. Unburned fuel is dumping into your exhaust system, which quickly overheats and destroys the catalytic converter. Pull over and turn off the engine immediately to prevent a $2,000+ repair bill.

What are common mistakes when diagnosing P0304?

The biggest mistake is replacing parts without confirming the diagnosis via a free coil swap test. Another error is ignoring freeze frame data, which reveals if the misfire only occurs under specific conditions like cold starts. Finally, mechanics often overlook fuel injectors or vacuum leaks when ignition fixes fail.

Why would P0304 appear only when the engine is cold?

Cold-start misfires frequently point to carbon buildup on intake valves, especially in direct-injection engines. They also indicate a leaky fuel injector dripping fuel overnight or a weak ignition coil that stabilizes as engine heat increases. Check freeze frame data to confirm the exact temperature when the code triggered.

Key Takeaways

• Code P0304 indicates cylinder 4 is misfiring, causing a 5-15% drop in fuel economy, rough idling, and engine hesitation.
• Stop driving immediately if the check engine light flashes; this indicates raw fuel is entering the exhaust and will destroy your $1,500+ catalytic converter in minutes.
• Perform a free 15-minute 'swap test' by moving the cylinder 4 ignition coil to cylinder 1; if the code changes to P0301, you confirmed a bad coil.
• Replace the ignition coil and spark plug first, as these two components resolve over 70% of isolated P0304 misfire codes.
• Check for manufacturer-specific flaws, such as GM's Active Fuel Management lifter failures on 5.3L V8s or Honda's 1.5L Turbo fuel dilution issues, before authorizing deep engine teardowns.