P0309
Cylinder 9 Misfire DetectedP0309 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 9 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 P0309 means
P0309 indicates the ECU has detected a misfire in cylinder 9 — a combustion event where the air/fuel mixture in that cylinder failed to ignite completely or at all. The ECU identifies misfires by monitoring crankshaft deceleration via the CKP sensor; each firing event produces a slight acceleration pulse, and a cylinder that consistently fails to contribute stands out as a rotational speed dip at the expected firing interval. P0309 is only possible on engines with nine or more cylinders, most commonly V10 and V12 platforms (such as the BMW M5 S85, Lamborghini V10, and Dodge Viper V10) or unusual firing-order numbering schemes.
Severity depends on frequency. A single detected misfire event may store the code and illuminate the MIL as a steady lamp. If the misfire rate is high enough to risk catalytic converter damage from unburned fuel igniting in the catalyst, the MIL will flash — a flashing MIL for a misfire code is an active warning that unburned hydrocarbons are currently entering and overheating the catalyst. Continued driving under a flashing MIL risks permanent catalyst destruction and secondary damage to the oxygen sensors.
Causes range from ignition (spark plug, coil, HT lead) and fuel delivery (injector) faults specific to cylinder 9, to loss of compression from a mechanical issue in that bore. The access challenge on V10/V12 engines — cylinder 9 is typically deep in the vee — means diagnosis benefits from a methodical swap-and-retest approach before any disassembly.
Common causes
Most-frequently reported root causes when P0309 is logged.
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1
Fouled, worn, or incorrectly-gapped spark plug on cylinder 9 preventing reliable ignition.
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2
Weak or failed ignition coil (or coil-on-plug unit) assigned to cylinder 9.
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3
Damaged or high-resistance HT lead connecting the coil to the spark plug on cylinder 9.
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4
Clogged or stuck-open fuel injector on cylinder 9 causing a lean or flooded mixture.
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5
Low compression in cylinder 9 from worn piston rings, a damaged valve, or a leaking head gasket.
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6
Vacuum leak at the intake manifold gasket or port on cylinder 9 causing a locally lean mixture.
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7
Wiring fault (open circuit, short, or poor connector) on the cylinder-9 coil or injector driver circuit.
Symptoms drivers notice
How to diagnose P0309
A typical diagnostic flow when this code is present.
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1
Scan for all DTCs; confirm P0309 and note whether additional cylinder-specific codes are present that might indicate a shared rail or coil driver fault.
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2
Check whether the MIL is flashing — a flashing light means stop driving as soon as safely possible to avoid catalyst damage.
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3
Perform a cylinder contribution test with a scan tool to confirm cylinder 9 is actually the misfiring cylinder and quantify misfire counts.
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4
Swap the cylinder-9 spark plug with a plug from a known-good cylinder and re-run; if the misfire follows the plug, replace it.
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5
Swap the cylinder-9 coil pack with one from a known-good cylinder and re-run; if the misfire moves, replace the coil.
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6
With the coil and plug confirmed good, perform an injector balance test or swap to check fuel delivery to cylinder 9.
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7
If ignition and fuel delivery are normal, perform a compression test on cylinder 9; low compression warrants further mechanical investigation (leak-down test, visual inspection).
Related powertrain codes
Frequently asked questions
Why does P0309 only appear on certain engines?
OBD-II reserves individual cylinder misfire codes P0301–P0312 for cylinders 1–12 in sequence. P0309 therefore only applies to engines with at least 9 cylinders — typically V10 or V12 engines, though some manufacturers use non-sequential cylinder numbering that can place cylinder 9 on a smaller engine.
What does a flashing MIL mean for a misfire code?
A flashing MIL signals that the misfire rate is high enough to force unburned fuel into the catalytic converter, which can overheat and permanently damage the catalyst. Stop driving as soon as it is safe to do so; continued operation risks a very expensive repair.
Can I just replace the spark plug and clear the code?
Replacing the spark plug is the logical first step and resolves many cases, but always confirm the misfire has stopped (via scan tool misfire counters) before declaring it fixed. If the plug was not the cause, the code and misfire will return quickly.
Could a single misfire event cause P0309?
The ECU requires a pattern of misfires within a set number of engine revolutions before storing the code, so a single random event usually does not set it. Persistent or recurring misfires — even at low rates — will eventually trigger the code and, at higher rates, the flashing MIL.
Disabling P0309 in software
RaceTune can permanently disable P0309 — 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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