P0303
- Powertrain
- Generic (SAE)
- Emissions
Get it looked at now
Raw fuel passing an unburned cylinder into the catalytic converter can overheat and destroy it, and a flashing MIL means that damage is in progress. Beyond the converter, the hesitation and power loss can be a hazard when merging or climbing. A cylinder-specific misfire also sometimes indicates a valve or head gasket problem that gets more expensive the longer it runs.
- Safe to drive
- Flashing light means stop driving and arrange a tow. Steady light with rough running: drive gently and briefly, straight to a shop.
Cylinder 3 Misfire Detected
The computer detected repeated misfires on cylinder 3 specifically. It has identified where combustion is failing but not why. Because the code names one cylinder, the cause is almost always something dedicated to that cylinder rather than a shared engine problem.
The ECM watches crankshaft velocity between power strokes. When the crank fails to accelerate as expected at cylinder 3's position in the firing order, that event is counted as a cylinder 3 misfire. Only three things stop a cylinder from firing properly: no usable spark, wrong or missing fuel, or a cylinder that cannot hold and compress the charge. Diagnosis is the process of eliminating those three in the cheapest order.
How the car detected itThe misfire monitor runs a short window of roughly 200 crank revolutions to catch catalyst-damaging misfire rates, which flash the MIL, and a longer window of roughly 1000 revolutions to catch emissions-threshold misfire, which sets a steady MIL. Rates are tallied per cylinder from crank sensor data, so the ECM can name cylinder 3 without any sensor in that cylinder. Rough roads can occasionally produce false counts, which is why some ECMs suspend the monitor when wheel speed data suggests it.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Flashing light means stop driving and arrange a tow. Steady light with rough running: drive gently and briefly, straight to a shop.
- Urgency
- Get it looked at now
- Will it pass emissions
- No
- If ignored
- Fuel economy and driveability degrade, and the catalytic converter takes cumulative heat damage that eventually restricts the exhaust and starves the engine of power. Unburned fuel washes the cylinder wall and dilutes engine oil, accelerating wear. If the root cause is mechanical, such as a valve that is no longer sealing, the damage progresses and the repair moves from a coil to a cylinder head.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Check engine light on or flashing
- Rough idle and uneven running
- Power loss, hesitation under acceleration
- Increased fuel consumption
- Possible smell of unburned fuel from exhaust
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Ignition coil for cylinder 3 failed | Very common | — |
| 2 | Spark plug for cylinder 3 worn or fouled | Very common | — |
| 3 | Cylinder 3 injector clogged | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Whether the malfunction light is flashing
A flashing light changes the answer to whether the car should be driven at all. It signals a misfire rate high enough to damage the catalyst immediately.
Whether any other misfire codes or P0300 are stored alongside it
P0303 alone points at cylinder 3 hardware. P0303 with several other cylinders or a random misfire code redirects the diagnosis toward shared causes: fuel pressure, a vacuum leak, timing or a failing crank sensor.
Which physical cylinder is number 3 on this engine, and on which bank
On a V6 or V8, cylinder 3 may be on either bank depending on the manufacturer's numbering. Confirming this before removing anything saves real time.
Fuel trims on the bank containing cylinder 3
A misfiring cylinder pushes unburned oxygen past the oxygen sensor, which reads as lean. Recognizing that this trim reading is a consequence, not a cause, prevents an expensive detour into fuel system diagnosis.
Recent maintenance and how long the plugs have been in
Plugs beyond their service interval, or aftermarket plugs of the wrong heat range or gap, are a frequent and cheap answer.
Verify the misfire is genuinely isolated to cylinder 3
Tools Scan tool with per-cylinder misfire data
Read per-cylinder misfire counters with the engine running at idle and at a steady 2000 rpm. Note whether counts appear only when cold, only under load, or continuously.
ExpectedCounts overwhelmingly on cylinder 3. Significant counts elsewhere reframe the diagnosis as a shared cause.
Swap the cylinder 3 coil with a healthy cylinder and re-test
Tools Hand tools, scan tool
Move the coil, clear the codes, and drive or idle until the misfire returns. Note which cylinder the code names the second time. On engines with coil packs shared between paired cylinders, the swap logic differs, so identify the ignition system type first.
ExpectedCode moving to the swapped cylinder confirms a failed coil. Code staying on cylinder 3 rules the coil out.
Pull and read the cylinder 3 spark plug against a healthy one
Tools Spark plug socket, gap gauge, torque wrench
Compare color, gap, electrode wear, insulator cracks and carbon tracking on the ceramic. Note whether the plug is dry, fuel-wet, oil-wet, or steam-cleaned.
ExpectedTan or gray and dry. Oil fouling suggests valve stem seals or rings, a suspiciously clean plug suggests coolant intrusion, fuel-wet suggests spark failure.
Confirm spark quality under compression
Tools Adjustable spark tester
A coil that sparks fine in open air can fail under cylinder pressure. Use an adjustable spark tester rather than grounding a plug against the block, which proves very little and risks the ignition module.
ExpectedA strong blue spark that holds at the tester's widest usable setting, matching what the other cylinders produce.
Test fuel delivery to cylinder 3
Tools Stethoscope or noid light, multimeter
Listen for an even injector click, compare injector resistance across the whole set, and confirm the ECM drive signal is present. If the engine allows it, swap the cylinder 3 injector with a neighbor and see whether the misfire follows.
ExpectedInjectors matched in resistance and clicking evenly. Compare within the engine rather than to a generic specification.
Inspect the coil circuit and the plug well
Tools Multimeter, inspection light or borescope
Check supply voltage, ground and trigger signal at the cylinder 3 coil connector, and look for oil in the spark plug tube. Oil in the well breaks down the boot's insulation and causes a misfire that returns weeks after new parts go in.
ExpectedFull supply voltage, clean ground, trigger present, and a dry plug well.
Compression and leak-down test cylinder 3
Tools Compression gauge, leak-down tester
If the misfire will not move with the coil, plug or injector, test the cylinder mechanically. Compare readings across cylinders and use leak-down to locate the escape path.
ExpectedWithin roughly 10 to 15% of the other cylinders. Air at the exhaust indicates a burnt exhaust valve; at the intake, an intake valve; at the oil filler, rings; bubbles in the coolant, a head gasket.
Check valvetrain and timing if compression is low
Tools Borescope, feeler gauges where applicable, service information
A collapsed hydraulic lifter, a worn cam lobe, an out-of-adjustment valve clearance on engines with solid lifters, or a chain that has jumped can all present as one weak cylinder. Confirm valve motion before condemning the head.
ExpectedBoth valves on cylinder 3 opening and closing through their full travel, valve clearance within specification where the engine is adjustable, and cam timing correct at the manufacturer's reference position. A collapsed lifter, worn lobe or tight clearance found here explains the low compression; a valvetrain that checks out redirects the diagnosis to the valve seats or the head gasket. If cam timing is found out by a tooth or more, correct that before drawing any conclusion from compression numbers - mistimed cams read low on every cylinder.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the ignition coil for cylinder 3 | The misfire code moving to the other cylinder after the coil is swapped | $130–$400 | DIY |
| Replace the spark plugs as a full set | The misfire following the plug on a swap, or worn, cracked, fouled or incorrectly gapped plugs found on inspection | $90–$450 | DIY |
| Clean or replace the cylinder 3 fuel injector | Injector not clicking, resistance out of line with the set, or the misfire following the injector on a swap | $180–$650 | Hard |
| Repair the coil supply, ground or trigger wiring | Voltage, ground or trigger signal missing at the cylinder 3 coil connector | $100–$350 | DIY |
| Replace valve cover gasket and plug tube seals to stop oil entering the plug well | Oil found pooled around the cylinder 3 plug or boot | $180–$650 | DIY |
| Replace a collapsed lifter or address valvetrain wear | Low compression on cylinder 3 traced to a valve not opening or closing fully on inspection | $500–$2,500 | Shop |
| Cylinder head repair for a burnt or leaking valve | Low compression with leak-down air audible at the intake or exhaust | $1,200–$4,000 | Shop |
| Head gasket replacement | Low compression with coolant loss, bubbles in the coolant, or combustion gases detected in the cooling system | $1,400–$3,500 | Shop |
US independent-shop ranges, parts plus labor. Most of these finish inside the first two entries. Access dominates plug and coil labor: a rear-bank cylinder on a transverse V6 can require intake removal and cost several times what the same job costs on an inline four. Budget 100 to 200 dollars for diagnosis and another 100 to 200 for compression and leak-down testing if the misfire proves stubborn. Prolonged misfire that destroys the catalytic converter adds 800 to 2500 dollars.
- Buying a coil and a plug at the same time so the actual cause is never learned, which matters when the misfire comes back.
- Grounding a plug against the block to check spark. It proves almost nothing about spark under compression and can damage the ignition driver.
- Treating the lean fuel trim that accompanies a misfire as the root cause and replacing oxygen sensors.
- Assuming cylinder 3 is on the same bank as cylinder 1. On V engines that depends entirely on manufacturer numbering.
- Replacing an injector because it is dirty, without ever testing whether the misfire follows it.
- Skipping the compression test after coil, plug and injector all come back good, and continuing to replace parts instead.
- Driving a flashing-MIL vehicle to save a tow fee, which routinely costs more than the tow in converter damage.
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
Found something wrong? Tell us — we publish corrections.