P0303 – Misfire No.3 cylinder

Last updated: April 10, 2026

P0303 means cylinder 3 is not firing correctly, so the engine may run rough and lack power. You may feel a shake at idle, a stumble on acceleration, or a flashing MIL under load. The PCM sets this code when it detects a misfire pattern tied to cylinder #3, not when it proves a bad part. According to many manufacturers’ factory diagnostic data, this code indicates the PCM has identified a cylinder-specific misfire that can raise emissions and catalyst temperature. Treat P0303 as a starting point. Confirm spark, fuel, compression, and air delivery before replacing components.

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P0303 Quick Answer

P0303 points to a misfire on cylinder #3. Start by verifying ignition output and plug condition on cylinder 3, then confirm fuel delivery and cylinder mechanical health with test results.

What Does P0303 Mean?

P0303 code means the PCM detected a misfire on cylinder number 3. In real terms, the crankshaft slows slightly when #3 should contribute power. The PCM counts those speed changes and flags cylinder 3 when it sees a repeated pattern. This matters because a true misfire can overheat the catalytic converter and damage it fast.

Technically, the PCM uses the crankshaft position signal to monitor small rotational speed variations each firing event. It then assigns those variations to a specific cylinder based on engine position data. P0303 does not name a failed part. It only identifies the cylinder where the misfire shows up, so you must confirm whether the root cause is ignition, fueling, air, or compression.

Theory of Operation

During normal operation, each cylinder produces a consistent torque pulse after the spark event. The PCM watches crankshaft speed changes and compares them to expected patterns. When combustion stays stable, misfire counters remain low and the catalyst stays protected.

With a cylinder 3 misfire, combustion energy drops or disappears on that cylinder. Ignition faults, incorrect fuel delivery, air dilution, or low compression can all create that result. The PCM detects the resulting crankshaft deceleration and logs P0303 when the events repeat enough to exceed its internal criteria.

Symptoms

P0303 symptoms usually show up first as a drivability complaint, then as a warning light.

• Check engine light (MIL) on, sometimes flashing during heavy load or acceleration
• Rough idle with a noticeable shake that often feels “rhythmic”
• Hesitation or stumble on tip-in, merging, or climbing grades
• Reduced power with poor throttle response, especially under load
• Fuel smell from the exhaust or black soot at the tailpipe in some cases
• Hard start or extended crank when the misfire occurs during cranking
• Poor fuel economy due to incomplete combustion and fuel trim corrections

Common Causes

• Ignition coil output weak on cylinder 3: Low coil energy or internal breakdown causes inconsistent spark under load, so the crankshaft speed change matches a No.3 misfire pattern.
• Spark plug fouled, worn, or incorrect gap at cylinder 3: A plug that cannot fire reliably leaves the mixture unburned, which the PCM flags as a cylinder-specific misfire.
• Fuel injector on cylinder 3 restricted or electrically faulty: A clogged nozzle or an injector with poor power/ground control delivers the wrong fuel mass and creates a lean or unstable burn on that cylinder.
• Carbon buildup on intake valves (GDI/direct injection engines): on GDI/TGDI/FSI/EcoBoost/Skyactiv-G engines, fuel does not wash the intake valves — carbon accumulates on valve faces and stems over time, disrupting airflow and causing cylinder-specific or random misfires, especially at idle or light load. On vehicles over 60,000 miles this is one of the most common real-world misfire causes. Intake valve cleaning (walnut blasting or chemical decarbonisation) is required — replacing spark plugs or ignition coils will not fix it.
• Vacuum leak local to cylinder 3 runner: A leaking intake gasket, cracked runner, or PCV port near No.3 leans that cylinder more than the others and triggers a cylinder 3 misfire.
• Compression loss on cylinder 3: A burned valve, worn rings, or head gasket leak reduces effective compression and slows combustion, which shows up as a misfire on that cylinder.
• Wiring or connector fault at coil or injector for cylinder 3: High resistance, terminal spread, or intermittent opens reduce coil or injector performance even when continuity tests “look good.”
• Fuel quality or fuel pressure delivery problem that shows up first on No.3: Low fuel pressure, contaminated fuel, or a weak pump can create lean misfires, and cylinder-to-cylinder variation can make No.3 the first one detected.

Diagnosis Steps

Tools you need: a scan tool with Mode $06 misfire data and live data, a DVOM for voltage-drop testing, and basic hand tools for plug and coil access. Add a fuel pressure gauge if service information allows. A spark tester and noid light help on older setups. A borescope and compression gauge make the diagnosis faster.

1. Confirm P0303 as pending or confirmed/stored. Record freeze frame data for RPM, load, coolant temperature, fuel system status (open/closed loop), STFT/LTFT, vehicle speed, and battery voltage. Use this to duplicate the exact conditions that set the fault.
2. Check for companion DTCs before touching parts. Pay attention to P0171/P0174 (lean), P0420 (catalyst), P035x (coil circuit), injector circuit codes, and crank/cam sensor codes. Those codes change the test path and can explain why cylinder 3 misfires.
3. Inspect fuses and power distribution for the ignition coils and injectors. Verify the correct fuse feeds and relay outputs with the key on. Do this before any ECU testing so you do not chase a “bad coil” that lacks power.
4. Verify PCM and engine grounds with a voltage-drop test under load. Command a load or run the engine and measure ground drop from the component ground to battery negative. Keep ground drop under 0.1V with the circuit operating, or repair the ground path.
5. Perform a focused visual inspection of cylinder 3 components and harness routing. Look for oil in the plug well, coolant intrusion, broken locks, terminal push-out, and rub-through near the valve cover. Tug-test the coil and injector connectors and check for terminal spread.
6. Use live data and Mode $06 to verify the misfire stays on cylinder 3. Watch misfire counters at idle and at the freeze-frame RPM and load. If misfires move to other cylinders, you likely have a shared cause like fuel pressure, air metering, or vacuum leaks.
7. Determine if the engine uses direct injection (GDI/DI). Check the underhood label, service data, or injector location. DI injectors mount in the cylinder head and spray into the combustion chamber, not the intake port. If the engine is DI and has 60,000+ miles, inspect intake valves with a borescope through the intake port before condemning coils, plugs, or injectors.
8. Test the ignition system on cylinder 3 without guessing. Swap the coil and plug from cylinder 3 with a known-good cylinder. Clear codes and recheck misfire counters. If the misfire follows the swapped part, you identified the suspect component or its connection.
9. Check injector function on cylinder 3. Listen with a stethoscope for a consistent click. If access allows, measure injector resistance and compare to the other cylinders. Perform an injector balance test or a power-balance test with a scan tool if available to confirm fuel delivery differences.
10. Check for a vacuum leak that targets cylinder 3. Use smoke testing at the intake, or use propane/brake-clean method only with proper safety precautions. Focus on intake gasket areas and PCV ports near the No.3 runner. Verify trims and misfire counters respond to the test.
11. Verify mechanical health if ignition and fuel tests do not isolate the cause. Run a compression test and, if needed, a cylinder leak-down test on cylinder 3. Compare results to adjacent cylinders. Mechanical faults can mimic ignition or injector problems and will not respond to parts swaps.
12. Confirm the repair under the same conditions as the freeze frame. Use a scan tool snapshot during a road test to capture live data when the concern usually occurs. Remember the difference: freeze frame shows when the DTC set, while a snapshot captures intermittent events during diagnosis.

Professional tip: Use Mode $06 misfire data after each change. It shows whether cylinder 3 improved before the MIL logic runs. That prevents unnecessary parts replacement and saves test-drive time.

Possible Fixes

• Repair power/ground feeds or high-resistance connections: Clean and tighten ground points, repair damaged wiring, and correct terminal fit issues found during voltage-drop and connector checks.
• Replace the spark plug on cylinder 3 (and correct the root cause of fouling): Install the correct plug type and gap, then address oil intrusion, coolant leaks, or rich/lean conditions that caused fouling.
• Replace the ignition coil on cylinder 3 after confirmation: Replace only after a swap test or waveform test proves the misfire follows the coil or the coil output is unstable.
• Service or replace the cylinder 3 fuel injector after testing: Restore proper fuel delivery based on balance testing, control circuit checks, or confirmed restriction.
• Intake valve cleaning on DI/GDI engines: Perform walnut blasting or approved chemical decarbonisation when borescope inspection confirms heavy valve deposits and symptoms match idle or light-load misfire.
• Repair intake air leaks: Replace a leaking intake manifold gasket, cracked runner, or PCV-related leak that drives a cylinder 3 lean condition.
• Mechanical repair for compression loss: Correct valve, ring, or head gasket faults when compression or leak-down results confirm a mechanical cause.

Can I Still Drive With P0303?

You can sometimes limp the vehicle with a P0303 code, but you should not “keep driving it” as normal. A cylinder 3 misfire can surge, buck, or stumble during acceleration. That can create a traffic-safety problem when you merge or pass. Continued driving also dumps unburned fuel into the exhaust. That overheats and damages the catalytic converter fast, especially if the misfire flashes the MIL. If the engine shakes at idle, lacks power, or the MIL flashes, stop driving and tow it. If the misfire feels mild and the MIL stays steady, drive only as needed to reach a repair location and avoid heavy throttle.

How Serious Is This Code?

P0303 ranges from an inconvenience to a converter-killing fault, depending on misfire severity and load. A light, occasional misfire may only cause a rough idle and reduced fuel economy. A hard misfire under load can stall the engine, reduce power sharply, and make the vehicle unsafe in traffic. The biggest financial risk comes from catalyst overheating and meltdown. That repair often costs far more than fixing the root cause. Treat a flashing MIL as severe and immediate. Also take this code seriously on engines with direct injection and higher mileage, because intake valve carbon can create chronic misfires that worsen over time.

Common Misdiagnoses

The most common mistake is swapping the ignition coil or spark plug on cylinder 3 without proving the misfire follows the part. Techs also skip basic electrical checks, then miss a coil power feed issue, poor ground, or connector pin fit that only fails under vibration. Another repeat failure comes from ignoring fuel trim data and vacuum leaks near one intake runner, which can lean out only cylinder 3. On GDI engines, many people replace coils, plugs, and injectors while the real cause is intake valve carbon buildup that disrupts airflow at idle and light load. Avoid wasted spending by confirming the misfire with Mode $06 or misfire counters, then verifying spark, fuel, compression, and air for that cylinder.

Most Likely Fix

The most frequent confirmed repair direction is correcting a cylinder 3 ignition problem, but only after you verify power, ground, and command integrity at the coil and prove the misfire follows the coil or plug. The other high-percentage direction, especially on high-mileage direct-injection engines, is intake valve carbon removal after borescope confirmation through the intake port. Either way, complete the repair verification with a road test under the enable conditions that allow the misfire monitor to run, and confirm no pending or confirmed misfire returns.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.