P0305
Cylinder 5 Misfire DetectedP0305 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 5 Misfire Detected. It is logged by the engine control unit when the misfire monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0305 means
P0305 is a generic OBD-II code indicating the PCM has detected a misfire in cylinder 5. The PCM uses the crankshaft position (CKP) sensor to measure the angular velocity of the crankshaft; each firing cylinder should produce a measurable acceleration pulse. When cylinder 5 fails to contribute this pulse within a calibrated tolerance — indicating incomplete or absent combustion — the PCM logs P0305 and illuminates the MIL. A steady MIL indicates an intermittent or low-rate misfire; a flashing MIL signals a severe, catalyst-damaging misfire rate currently in progress.
Cylinder 5 is present in five-cylinder engines (I5), inline-six engines (I6), V8 engines (V8 Bank 2 — the 5th cylinder in firing order), V10 engines, and some V12 configurations. The physical location and accessibility of cylinder 5 vary significantly by engine layout and vehicle model. P0305 is a combustion-result code — it does not specify which subsystem failed — so the diagnosis must systematically evaluate ignition, fuelling, air delivery, and mechanical compression to locate the root cause.
The most frequent causes across platforms are worn or fouled spark plugs, failed ignition coils, and clogged fuel injectors. Mechanical causes such as low compression or a burned valve are less common but must be ruled out, particularly on high-mileage engines or when ignition and fuel components test serviceable.
Common causes
Most-frequently reported root causes when P0305 is logged.
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1
Worn, fouled, or incorrectly gapped spark plug on cylinder 5.
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2
Failed or weak ignition coil (or coil pack) for cylinder 5.
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3
Clogged or faulty fuel injector on cylinder 5 disrupting fuel delivery.
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4
Vacuum leak on the cylinder 5 intake runner causing a lean air-fuel mixture.
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5
Low compression in cylinder 5 from worn piston rings, a damaged valve, or a blown head gasket.
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6
Damaged or corroded wiring/connector to the cylinder 5 ignition coil or injector.
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7
Incorrect ignition timing or a stretched timing chain affecting cylinder 5's combustion event.
Symptoms drivers notice
How to diagnose P0305
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scan tool, confirm P0305, and note any companion codes (fuel trim codes, additional misfire codes, ignition circuit codes).
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2
Remove and inspect the spark plug on cylinder 5 — check for fouling, cracks, erosion, and correct gap; replace if in doubt.
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3
Swap the cylinder 5 ignition coil with the coil from an adjacent cylinder and re-scan: if the misfire code follows the coil to its new location, the coil is faulty.
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4
Test the cylinder 5 fuel injector using scan tool live data for drop tests or a noid light for pulse confirmation; inspect the injector connector for corrosion.
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5
Perform a compression test on cylinder 5 — specification varies by engine but generally 140–200 psi; a reading more than 25% below adjacent cylinders is significant.
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6
Check for vacuum leaks at the cylinder 5 intake runner using a smoke machine or careful propane enrichment test.
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7
If all above components test normal, check ignition timing, inspect the timing chain/belt for wear, and review engine mechanical condition.
Related powertrain codes
Frequently asked questions
Will a flashing check engine light from P0305 damage my catalytic converter?
Yes, rapidly. A flashing MIL means the misfire rate is high enough that unburned fuel is entering the exhaust and igniting in the catalytic converter, causing it to overheat. Catalytic converter replacement is expensive — stop driving and diagnose immediately when the MIL flashes.
How do I know if my engine even has a cylinder 5?
Any engine with five or more cylinders has a cylinder 5: straight-five (I5), inline-six (I6), V8, V10, and V12 engines all include cylinder 5 in their numbering. Four-cylinder engines do not; P0305 on a four-cylinder is a sign of a scan tool error or module communication fault.
The misfire is only present when cold — does that change the diagnosis?
Cold-start misfires that clear as the engine warms most often point to a marginal spark plug, a coil with weakened output when cold, or injector deposits that clear as fuel warms up. Start with the spark plug and coil. Compression issues can also manifest more at cold start due to reduced ring seal when cold.
Can I swap ignition coils between cylinders to test without causing harm?
Yes, swapping coils is a standard and safe diagnostic technique on coil-on-plug (COP) systems, provided you do not damage the connectors during removal. After swapping, run the engine briefly and re-scan — if the P0305 becomes a P0302 (or whichever cylinder the coil moved to), the coil is confirmed faulty.
Disabling P0305 in software
RaceTune can permanently disable P0305 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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