OBD-II Code P0303: Cylinder 3 Misfire Detected

What Does P3303 Mean?

Code P0303 signifies your vehicle's Powertrain Control Module (PCM) detected that cylinder 3 is failing to ignite its air-fuel mixture correctly. An engine misfire occurs when combustion is incomplete, causing the engine to run rough, lose power, vibrate, and produce harmful emissions.

Technical definition: The official SAE/OBD-II definition for P0303 is "Cylinder 3 Misfire Detected." 🎬 Watch: How to diagnose spark plugs and ignition coils The PCM constantly monitors the rotational speed of the crankshaft. When it detects a slowdown corresponding specifically to the power stroke of cylinder 3, it flags a misfire event.

Can I Drive With P3303?

Yes, But With Caution. Driving is not recommended. If the check engine light is solid, you can drive short distances to a repair shop, but a persistent misfire causes long-term engine damage. If the light is flashing, pull over and stop the engine immediately. A flashing light indicates a severe misfire dumping unburned fuel into the exhaust, which destroys the catalytic converter in a matter of miles, turning a $250 repair into a $2,000+ catastrophe.

Common Causes

• Worn or Fouled Spark Plug (Very Common) — This is the primary cause. Spark plugs wear out over time, leading to a weak spark. They also become fouled by oil or carbon, or suffer from an incorrect gap, preventing proper ignition.
• Faulty Ignition Coil or Coil Pack (Very Common) — The ignition coil converts low battery voltage to the high voltage needed for a spark. The coil for cylinder 3 fails frequently due to heat and engine vibration, delivering insufficient energy to the spark plug.
• Clogged or Faulty Fuel Injector (Common) — A fuel injector becomes clogged with carbon deposits or fails electronically, preventing the correct amount of fuel from spraying into cylinder 3. This disrupts the precise air/fuel ratio required for combustion.
• Carbon Buildup on Intake Valves (Common) — On Gasoline Direct Injection (GDI) engines, carbon deposits accumulate heavily on the back of the intake valves. This buildup acts like a sponge, absorbing fuel and obstructing airflow, causing misfires during cold starts.
• Vacuum Leak (Less Common) — A cracked vacuum hose or failed intake manifold gasket near cylinder 3's runner allows unmetered air into the engine. This creates a lean air/fuel mixture for that specific cylinder, causing it to misfire.
• Leaking Valve Cover Gasket (Less Common) — A failed spark plug tube seal allows engine oil to flood the spark plug well. This oil saturates the ignition coil boot and spark plug, shorting out the electrical current and causing a misfire.
• Low Engine Compression (Rare) — This indicates a serious internal mechanical failure. Causes include a burnt exhaust valve, worn piston rings, a blown head gasket, or a failed valvetrain component (like a collapsed lifter or worn camshaft lobe).

Symptoms

• Check Engine Light On or Flashing — A solid light is the first alert. A flashing light signals a severe, catalyst-damaging misfire requiring an immediate and safe stop.
• Rough Idle and Engine Vibration — The engine shakes or shudders, especially at idle or low speeds. The vehicle feels noticeably unbalanced.
• Hesitation and Reduced Power — The vehicle stumbles or jerks when pressing the accelerator. Overall engine power drops significantly, accompanied by poor fuel economy.
• Fuel Smell from Exhaust — A distinct smell of raw gasoline from the tailpipe indicates unburned fuel from cylinder 3 is passing directly into the exhaust system.
• Engine Stalling — When coming to a stop or operating under low load, the misfire causes the engine RPMs to drop low enough to stall completely.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace Spark Plugs (Full Set) — Parts: $40-$100, Labor: $100-$300, ~1.0 hr book time (DIY)
• Replace Ignition Coil (Cylinder 3) — Parts: $50-$150, Labor: $100-$250, ~0.5 hr book time (DIY)
• Replace Fuel Injector (Cylinder 3) — Parts: $100-$350, Labor: $300-$800, ~2.5 hr book time (Intermediate)
• Intake Valve Carbon Cleaning (Walnut Blasting) — Parts: $20-$50, Labor: $500-$1000, ~4.5 hr book time (Professional)
• Intake Manifold Gasket Replacement — Parts: $40-$120, Labor: $300-$600, ~2.8 hr book time (Professional)
• Valvetrain Repair (Rocker Arm / Lifter) — Parts: $200-$1500, Labor: $800-$3500, ~8.0 hr book time (Professional)

DIY vs Professional

• Replace Spark Plug — Beginner:
• Replace Ignition Coil — Beginner:
• Replace Fuel Injector — Beginner:
• Intake Valve Carbon Cleaning — Beginner:

Used vs. New Parts: Buying Guide

When a used part is worth it: Buying used ignition coils or fuel injectors is never recommended. These are wear items with limited lifespans. Installing a used part that fails quickly is a poor value proposition given the labor involved.

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

Donor quality checklist:

• For major mechanical components (cylinder heads), avoid parts from high-mileage vehicles.
• Ensure the part number matches exactly; incorrect OEM parts cause severe performance issues.
• Purchase from a reputable salvage yard offering at least a 30-day warranty.

Decision logic:

• If The part is an ignition coil or spark plug → Always buy new. The cost savings of a used part are negligible compared to the risk of a repeat failure.
• If The part is a fuel injector → Buy a new or professionally remanufactured OEM-quality part. Cheap aftermarket or used injectors are a common source of new problems.
• If The repair requires a major mechanical component (e.g., cylinder head) → A low-mileage used component from a reputable salvage yard is a viable option to save significant cost.

Warranty tradeoff: Used parts typically have a 30-day warranty. New aftermarket parts carry a 1-year warranty. New OEM parts offer the best guarantee of quality and fitment.

Worst-case if a used part fails: 300-800 if a used coil or injector fails after installation, requiring repeat labor and the purchase of another part.

What Happens If You Wait — Timeline

1. 0-1 week: Solid Check Engine Light appears. A slight rough idle is noticeable. Performance is largely normal during gentle driving. (MPG impact: 0-5%% · Added cost: $0)
2. 1 week - 2 months: Engine vibration at idle becomes pronounced. Hesitation occurs during acceleration. Fuel economy drops. The check engine light flashes during heavy acceleration. (MPG impact: 5-15%% · Added cost: $25-$100 in wasted fuel)
3. 2-4 months: A flashing check engine light becomes frequent. Unburned fuel regularly enters the exhaust, causing the catalytic converter's internal temperature to spike and melt its ceramic honeycomb structure. (MPG impact: 15-25%% · Added cost: $800-$2,500 (Catalytic converter replacement required))
4. 4+ months: The catalytic converter becomes fully clogged. Severe power loss occurs, the engine stalls, and exhaust smells like sulfur. Excessive backpressure damages exhaust valves. (MPG impact: 25-50%% · Added cost: $1,500-$3,500+ (Catalytic converter, O2 sensors, and engine damage))

Cost of Not Fixing It

• 0-1 month: Noticeable drop in fuel economy (5-15%), rough idle, and poor acceleration. (Added cost: 50-150)
• 1-6 months: Catalytic converter failure due to overheating from unburned fuel. This is the most significant and costly consequence. (Added cost: 1200-2800)
• 6+ months: Secondary damage, including fouled oxygen sensors, burnt exhaust valves, or accelerated wear on engine mounts due to severe vibration. (Added cost: 300-1000)

Diagnosis Steps

1. Read Codes and Freeze Frame Data
   Use an OBD-II scanner to confirm P0303. Check for accompanying codes (e.g., P0300, P0171, P0353) and analyze the freeze frame data to see the exact engine RPM, load, and temperature when the misfire occurred.
   Tools: OBD-II Scanner (Beginner)
2. Perform the Ignition Coil Swap Test
   Swap the ignition coil from cylinder 3 with the coil from cylinder 1. Clear the codes and drive. If the code returns as P0301, the coil is definitively faulty. This free test prevents replacing good parts.
   Tools: Basic socket set or screwdriver (Beginner)
3. Swap and Inspect the Spark Plug
   If the misfire stayed on cylinder 3, repeat the swap test with the spark plug. Move the plug from cylinder 3 to cylinder 2. If the code follows the plug (P0302), replace the plugs. Inspect the old plug for heavy carbon fouling, melted electrodes, or oil saturation.
   Tools: Spark plug socket, ratchet, extension (Intermediate)
4. Perform a Targeted Visual Inspection
   Inspect the wiring harness and connector going to cylinder 3's coil and injector for frayed wires or loose pins. Look for cracked vacuum lines near the cylinder 3 intake runner.
   Tools: Flashlight (Beginner)
5. Test the Fuel Injector
   Listen to the cylinder 3 injector with a mechanic's stethoscope; it should emit a sharp, consistent clicking. If silent, check for a pulsing ground signal from the PCM using a noid light. Test the injector's internal resistance with a multimeter (typically 11-14 ohms).
   Tools: Mechanic's stethoscope, multimeter, noid light set (Intermediate)
6. Check for Vacuum Leaks with a Smoke Test
   Connect a smoke machine to the intake manifold. Pressurized smoke visibly pours out of any cracks in hoses or failed intake gaskets, instantly pinpointing the source of unmetered air causing a lean misfire.
   Tools: Smoke machine (Intermediate)
7. Analyze Fuel Trims and Mode $06 Data
   Using an advanced scanner, check short-term and long-term fuel trims (STFT/LTFT). Readings consistently above +10% confirm a vacuum leak or under-fueling. Check Mode $06 data to view raw misfire counts per cylinder in real-time.
   Tools: Advanced OBD-II Scanner (Advanced)
8. Perform a Compression Test
   If spark and fuel are confirmed good, check mechanical health. Remove all spark plugs and test cylinder 3's compression. Readings below 120 PSI, or a 15% variance from other cylinders, indicate internal engine failure. Stop DIY diagnosis and tow the vehicle to a professional shop.
   Tools: Compression tester (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-210°F (Fully warmed up, but frequently occurs during cold start warm-up (<160°F) on GDI engines.)
• RPM: 600-1200 (Idle or low RPM, where mixture stability is crucial and misfires are most noticeable.)
• Engine Load: 20-40% (Low load or light acceleration.)
• Vehicle Speed: 0-25 mph (Often occurs at idle or during stop-and-go city driving.)

Related Codes

• P0300 — Indicates a 'Random/Multiple Cylinder Misfire'. If paired with P0303, the problem is systemic. Diagnose low fuel pressure, a large vacuum leak, or bad gas before replacing cylinder 3 parts.
• P0171 or P0174 — These 'System Too Lean' codes point directly to a vacuum leak when paired with P0303. Unmetered air is entering near cylinder 3's intake runner, causing both the lean condition and the misfire.
• P0353 — Indicates 'Ignition Coil C (Cylinder 3) Primary/Secondary Circuit Malfunction.' The PCM has detected a hard electrical fault. The problem is definitively the cylinder 3 coil, its connector, or the wiring harness.

Climate & Environmental Factors

• Cold Weather: Cold temperatures exacerbate misfires on startup. Fuel atomizes poorly when cold, and carbon buildup on intake valves absorbs this poorly atomized fuel, creating a lean condition until the engine warms up.
• High Humidity: In humid conditions, turbocharged engines (like the Ford EcoBoost) build condensation in the intercooler. Under heavy acceleration, this pooled water is ingested by the engine, causing a sudden, violent misfire.
• High Altitude: At higher altitudes, air is less dense. While the PCM compensates, this exposes underlying weaknesses. A marginally performing ignition coil that works fine at sea level fails to bridge the spark gap at 8,000 feet.

How to Talk to a Mechanic About This Code

Say this: "I have a P0303 code for a cylinder 3 misfire. I'd like to schedule a diagnostic. Please start by swapping the ignition coil and spark plug from cylinder 3 to another cylinder to see if the code follows the part. Please call me with the results of that specific test before proceeding with any other repairs."

This signals you have done your research and directs the mechanic to perform a logical, no-cost diagnostic step first. It prevents them from immediately replacing multiple expensive parts and forces them to prove which part has failed.

Avoid saying:

• 'Just fix whatever's wrong'
• 'My check engine light is on, can you look at it?' (too vague — invites upsell)
• 'Whatever you recommend'

Questions to ask before authorizing the repair:

• Did the misfire code move to the other cylinder when you swapped the coil or the spark plug?
• What did the spark plug from cylinder 3 look like? Was it fouled with oil, wet with fuel, or just worn?
• If the swap test didn't isolate the problem, what is your next diagnostic step? Will you be testing the fuel injector or performing a compression test?
• Can you provide a written estimate with a full breakdown of parts and labor costs?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended only for in-warranty repairs or known, complex pattern failures specific to your vehicle's manufacturer.
  Best for: Vehicles still under powertrain or emissions warranty., Known manufacturer-specific mechanical failures (e.g., GM AFM lifters, Pentastar rocker arms) that have extended warranty coverage., Complex electrical issues requiring PCM software updates.
  Downsides: Significantly higher labor rates (1.5-2x) versus independent shops., May recommend replacing an entire assembly when only one component has failed. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for most out-of-warranty P0303 repairs. A competent independent shop easily diagnoses and fixes the common causes of this code at a reasonable price.
  Best for: Out-of-warranty vehicles where cost is a major factor., Standard diagnosis and repair of common causes like spark plugs, coils, and fuel injectors., 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 access to the very latest manufacturer service bulletins. (Typical cost: +0% vs. baseline)
• Chain Shop: Use with caution. Acceptable if you have already diagnosed the problem yourself. AVOID for initial diagnosis, as they frequently recommend unnecessary parts.
  Best for: Simple, pre-diagnosed part replacements (e.g., 'I know the coil on cylinder 3 is bad, please replace it').
  Downsides: Technician skill and diagnostic ability vary dramatically., High pressure to upsell services like fuel system cleanings or replacing all coils/plugs when only one is bad., Often lack advanced diagnostic tools for deeper mechanical problems. (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, seriously consider selling or trading in the vehicle. This is especially true if the diagnosis points to a major mechanical failure like low compression or camshaft damage.

• Car worth $4000, fix is $2000: Borderline — get a 2nd opinion before authorizing. This is a significant percentage of the car's value.
• Car worth $12000, fix is $1800: Fix it — this repair is well below the 50% threshold and makes economic sense.
• Car worth $2500, fix is $1500: Walk away — the repair cost is over 50% of the vehicle's value. It is not a sound investment.

What Scan Tool You Need for This Code

Minimum: A basic OBD-II reader that reads and clears codes is sufficient for the initial 'swap test' diagnostic. However, a scanner with Live Data is highly recommended.

A simple $20 code reader confirms the P0303 code but won't show crucial live data like fuel trims or live misfire counts per cylinder (Mode $06). This data confirms the misfire in real-time and points toward a fuel vs. spark issue before you pick up a wrench.

Budget: BlueDriver Pro (~$100) — Connects to your smartphone via Bluetooth. Provides the code, freeze-frame data, live data graphing, and access to Mode $06 misfire counts.

Mid-range: Foxwell NT510 Elite / Autel MaxiCOM MK808 (~$150) — Offers everything the budget picks do, plus manufacturer-specific diagnostics and bidirectional controls to test components like fuel injectors without removing them.

Professional: Autel MaxiSys MS906 / Launch X431 Series (~$500-1200) — Provides full bidirectional control, OE-level diagnostics, and advanced data graphing. These professional-grade tools diagnose the most obscure causes of a misfire, including PCM driver faults.

Rent vs buy: If this is a one-time diagnosis, auto parts stores like AutoZone have a tool loaner program where you can borrow an OBD-II scanner for free with a refundable deposit. Buy a scanner only if you plan to perform your own diagnostics regularly.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the P0303 code.
2. Perform a complete drive cycle to allow all readiness monitors to run.
3. Re-scan the vehicle to ensure the code has not returned as pending.

Drive cycle (~30 minutes): Cold start (sit for 8+ hours). Idle for 3 minutes with A/C and rear defroster on. Drive at a steady 55 mph for 10 minutes. Coast to a stop without braking. Drive in stop-and-go city conditions for 15 minutes.

Readiness monitors affected: Misfire Monitor, Catalyst Monitor, O2 Sensor Monitor

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

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors, guaranteeing an automatic emissions test failure.
• The code returns immediately if the root cause of the misfire was not correctly repaired.
• Driving only on the highway at a constant speed fails to complete the stop-and-go monitor tests.

Will This Fail Emissions / State Inspection?

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

• California: An active P0303 code is an automatic failure. After repair, all readiness monitors must be set to 'Ready' before a retest. The Misfire and Catalyst monitors must pass.
• New York: The NYS DMV inspection includes an OBD-II scan. A P0303 code causes an immediate failure of the emissions portion of the test.
• Texas: In the 17 counties requiring emissions testing, a P0303 code results in a failed test. The check engine light must be off and readiness monitors complete to pass.

Most Commonly Affected Vehicles

• Chevrolet/GMC Silverado, Tahoe, Suburban (5.3L/6.2L V8) (2010-2018) — A persistent P0303 is frequently caused by a collapsed Active Fuel Management (AFM) lifter, a major mechanical repair. Also check for loose spark plug wires per TSB #PIP4819A.
• Ford F-150 (3.5L EcoBoost) (2011-2017) — Suspect carbon buildup or moisture in the intercooler causing misfires under load. Cylinder 3 is the rear-most cylinder on the passenger side.
• Chrysler/Dodge/Jeep Various with 3.6L Pentastar V6 (2011-2021) — A P0303 code accompanied by a ticking noise strongly indicates a failed rocker arm bearing that has damaged the camshaft lobe. Accessing cylinder 3 requires removing the upper intake manifold.
• Volkswagen/Audi Jetta, Golf, A4 (2.0T TSI/FSI) (2009-2018) — Ignition coils are a high-failure item. The second most common cause is heavy carbon buildup on the intake valves due to the direct-injection design, leading to severe cold start misfires.
• Kia/Hyundai Forte, Soul, Elantra (1.6L/2.0L) (2012-2019) — Often caused by a faulty ignition coil. TSB 20-FL-001H outlines replacing coils and updating the ECU software to correct over-sensitive misfire detection logic.
• Nissan Altima, Maxima, Murano (3.5L VQ35DE) (2007-2018) — Ignition coils fail frequently. A leaking valve cover gasket also commonly causes oil to pool in the cylinder 3 spark plug tube, shorting the coil.

Manufacturer-Specific Notes

• General Motors (Chevrolet/GMC): On V8 engines with Active Fuel Management (AFM), a lifter for cylinder 3 collapses and sticks, causing a dead cylinder. This major mechanical failure is usually preceded by a loud ticking noise from the top of the engine.
• Volkswagen/Audi: Direct-injection (TSI/FSI) engines are highly susceptible to carbon buildup on intake valves. This causes cold-start misfires that a standard tune-up will never fix; it requires physical media blasting.
• Chrysler/Dodge/Jeep: The 3.6L Pentastar V6 is infamous for valvetrain issues. A P0303 code with an engine tick almost always points to a failed rocker arm bearing. Chrysler extended the left-side cylinder head warranty for some 2011-2013 models due to this.

Real Owner Stories

2015 Jeep Wrangler with 3.6L Pentastar

Check engine light was on with a consistent P0303 code and a slight misfire at idle. A previous shop replaced the plug, coil, and injector with no change and suggested a costly cylinder head repair.

What they tried:

1. Replaced spark plug, ignition coil, and fuel injector for cylinder 3.
2. A second shop performed a visual inspection under the valve cover.

Outcome: The second shop identified a failed rocker arm roller bearing on cylinder 3, a common Pentastar issue. The rocker arm was replaced, preventing camshaft damage and resolving the misfire.

Lesson: On Pentastar engines, a persistent single-cylinder misfire with a ticking noise is usually a failed rocker arm, not a burnt valve. Always inspect the valvetrain before approving a cylinder head replacement.

2015 Ford F-150 with 3.5L EcoBoost at 82K miles

Vehicle experienced severe stuttering and hesitation when accelerating. The check engine light came on and a scanner showed code P0303.

What they tried:

1. Flashed the custom tune back to the stock Ford tune; the problem remained.
2. Swapped the spark plug from cylinder 3 to cylinder 2.

Outcome: After swapping the spark plugs, the misfire code changed to P0302 (Cylinder 2 Misfire). This confirmed the spark plug was the culprit. All spark plugs were replaced, resolving the issue.

Lesson: Always perform the free 'swap test' with plugs and coils before buying new parts. Moving the suspected part to another cylinder to see if the code 'follows' it is the fastest way to confirm failure.

2012 Kia Soul with 1.6L at 145K miles

Car had a P0303 code with a noticeable rough idle and shaking when stopped in gear.

What they tried:

1. Replaced all spark plugs.
2. Replaced the ignition coil on cylinder 3.

Outcome: The P0303 code persisted after replacing ignition parts. The next diagnostic steps were checking for a clogged fuel injector or performing a compression test to check for mechanical issues.

Lesson: If a new spark plug and ignition coil do not fix the misfire, do not keep replacing parts. The problem lies deeper in the fuel system or is a mechanical engine fault. Systematic diagnosis is required.

How to Prevent This Code From Triggering

• Replace spark plugs at the manufacturer's recommended interval. (Every 60,000 to 100,000 miles.) — Worn spark plugs are the primary cause of misfires. As electrodes wear, the spark becomes weak and inconsistent, leading to incomplete combustion.
• Use Top Tier certified gasoline. (Every fill-up.) — Top Tier fuels contain higher detergent concentrations that prevent carbon deposits from forming on fuel injectors, ensuring a proper fuel spray pattern.
• Perform periodic fuel system cleaning. (Every 15,000 miles.) — Using a high-quality fuel system cleaner containing PEA (polyetheramine) removes existing deposits from injectors, restoring proper fuel flow.
• Adhere to recommended oil change intervals with high-quality synthetic oil. (Every 5,000 to 7,500 miles.) — Clean oil is crucial for hydraulic lifters and variable valve timing solenoids. Dirty oil causes these parts to stick, leading directly to misfires.

Frequently Asked Questions

Is it safe to drive with a P0303 code?

It is strongly advised not to. If the check engine light is flashing, pull over immediately. A flashing light means raw fuel is overheating your catalytic converter, which causes a failure costing $800-$2,500 to replace.

How much does it cost to fix a P0303 code?

The cost varies dramatically based on the root cause. A DIY spark plug replacement costs under $50, while a professional ignition coil replacement runs $200-$400. Major mechanical issues like a GM AFM lifter failure easily exceed $3,000.

Where is cylinder 3 on my engine?

Cylinder 3's location depends on your engine configuration. On inline engines, it is usually the third cylinder from the front belt drive. On V6 or V8 engines, consult your vehicle's service manual, as bank layouts vary widely by manufacturer.

I replaced the spark plug and coil, but P0303 came back. What now?

You have successfully ruled out the most likely culprits. The diagnostic path now moves to testing the fuel injector, checking for vacuum leaks, and performing a compression test. Stop replacing parts blindly and follow a systematic diagnostic process.

Can a bad O2 sensor cause a P0303 code?

Indirectly, yes. A faulty oxygen sensor sends incorrect data to the PCM, causing improper fuel mixture adjustments across an entire engine bank. While this usually triggers a P0300 random misfire, it sometimes contributes to a single-cylinder misfire.

Will a fuel additive or injector cleaner fix a P0303?

It is possible, but unlikely to be a permanent fix. If the misfire stems from a mildly clogged fuel injector, a high-quality fuel system cleaner temporarily improves flow. It will not fix a failed ignition coil, bad spark plug, or vacuum leak.

Can a P0303 code fix itself?

No, a misfire code indicates a hardware or software problem requiring intervention. The check engine light turns off if the misfire is intermittent, but the underlying issue persists. Ignoring it leads to worsening performance and more expensive secondary repairs.

Key Takeaways

• Code P0303 isolates your engine misfire specifically to cylinder 3, ruling out random misfire causes like a failing fuel pump.
• Replacing the cylinder 3 spark plug and ignition coil resolves over 70% of P0303 codes for under $200 in parts.
• A flashing check engine light means raw fuel is actively melting your catalytic converter; pull over immediately to prevent a $2,000+ repair bill.
• Perform a free 10-minute 'swap test' by moving the cylinder 3 ignition coil to cylinder 1; if the code changes to P0301, the coil is definitively bad.
• If ignition parts test good, check fuel trims for a vacuum leak or perform a compression test to rule out a $3,000+ internal engine failure.