OBD-II Code
P0309
Severity
Not yet reviewed
Nobody on our side has assessed this code yet, so we are not going to guess at how serious it is.
Can you drive it
We cannot say
Treat it as unknown until a shop reads it with live data. If the check-engine light is FLASHING, stop driving as soon as it is safe to — that means an active misfire, whatever the code.
Indicative cost (USD)
$90–$2,400
From this code's diagnostic notes. Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
P0309 means the engine control module detected a misfire on cylinder 9, specifically — meaning crankshaft deceleration events crossed the OBD-II threshold (typically 2% deviation for emissions-relevant misfire, 10% for catalyst-damaging) at the position in the firing order corresponding to that cylinder.
How a shop works this
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing.
Photograph the coils before you touch anything, and note which is which.
Cylinder numbering is not the same across manufacturers, and V6 and V8 bank layouts differ by maker — look up the firing order and bank layout for your engine before you move parts, or you can spend an hour swapping coils on the wrong bank.
Move the cylinder 9 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 9, clear the code, and run it until the misfire returns.
On a modern coil-on-plug engine the coil is the most common single answer to a cylinder-specific misfire, which is why it is the first thing to move — not because it is the cheapest part. A misfire that is there even at idle is the classic dead-coil pattern.
- The misfire follows the coil to its new cylinderThe coil is the fault. Replace it. If the engine has real mileage on it, the other coils are on the same clock.
- The misfire stays on cylinder 9The coil is fine. Look at the coil connector and the harness around it next — chafed and rodent-damaged connections at that plug are a common second answer — then pull the plug.
Read the plug you pulled, and check the boot for carbon tracking.
Black and sooty means it has been running rich. White or blistered means lean or too hot. Oil-wet points at valve seals or rings. Fuel-wet points at an injector holding open. A carbon-tracked boot arcs to the head and imitates a dead coil for the price of a new boot. On an older engine with a distributor, the cap, rotor and plug wires belong in this step too.
If spark is accounted for, go to fuelling, then to compression.
An injector balance or flow test comes first because it is quicker. Compression test second: a cylinder more than about 15% below its neighbours is mechanical, and no ignition part will fix it. A vacuum leak on this one runner sits alongside these — a leak big enough to affect every cylinder would be setting P0300 instead.
A flashing check-engine light means the misfire is putting raw fuel into the catalytic converter, which can destroy it quickly. Stop driving as soon as it is safe to. A steady light with a mild misfire is less urgent, but stop anyway if it shakes hard, loses power, overheats, or makes a noise it was not making before.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Likely causes
- Failed ignition coil (cylinder 9)Most common
- Damaged coil connector or harness at cylinder 9
- Worn spark plug or carbon-tracked boot (cylinder 9)
- Clogged or leaking fuel injector (cylinder 9)
- Low compression on cylinder 9
- Vacuum leak at the cylinder 9 intake runner
Read the full diagnostic procedureP0309 means the engine control module detected a misfire on cylinder 9, specifically — meaning crankshaft deceleration events crossed the OBD-II thre…
P0309 means the engine control module detected a misfire on cylinder 9, specifically — meaning crankshaft deceleration events crossed the OBD-II threshold (typically 2% deviation for emissions-relevant misfire, 10% for catalyst-damaging) at the position in the firing order corresponding to that cylinder.
Start cheapest-first
- pull the cylinder 9 coil and plug, inspect the plug for fouling (black soot = rich, white = lean, oil = valve seal/ring, melted electrode = lean lean knock), then swap coil 9 with coil 1 and clear codes — if the misfire follows to cylinder 1, you have a bad coil ($40-90 part).
- If it stays on 9, do a wet/dry compression test (a healthy V10/V8 reads 150-200 psi; a 30+ psi drop with oil bringing it back up = rings, no recovery = valves or head gasket), then a leak-down test to localize escape route.
- Don't skip checking the cylinder 9 injector resistance (typical 11-16 ohms; open or shorted injector misfires identically to ignition failure) and the wiring harness — rodent damage on rear-cylinder injector connectors is endemic on V10/V8 platforms.
- Caveat: a single misfire code on a high-mileage motor often masks a deeper pattern — always read freeze-frame for fuel trims, RPM, and load, and look for companion lean codes that point to vacuum leaks faking a misfire.
Vehicle-specific patternsFord 6.8L V10 (Excursion, E-350/450, F-250/350 Super Duty, 1999-2010) — cylinder 9 spark plug blowout is the signature failure; the two-piece plug de…
Ford 6.8L V10 (Excursion, E-350/450, F-250/350 Super Duty, 1999-2010) — cylinder 9 spark plug blowout is the signature failure; the two-piece plug design and shallow aluminum head threads (only 4 threads engaged pre-2008) eject the plug under combustion pressure, often taking the coil boot and head threads with it. Repair requires a Time-Sert or Big-Sert thread insert kit and pulling the doghouse on vans to reach the rear cylinders.
- 2008-2014Lamborghini Gallardo/Audi R8 5.2L V10 — coil pack failure on cylinder 9 is common. BMW S85 V10 (E60 M5, E63 M6, 2005-2010) — rod bearing wear shows up as cylinder 9/10 misfire under load before catastrophic failure; if compression is good and coil/plug/injector check out, pull the oil pan and inspect bearing shells.
Estimated repair$90 to $2,400.
Look up another code
P0309 — frequently asked
What does a P0309 code mean?
See deep dive below for full diagnostic context.
What causes a P0309 code?
Common causes: Failed ignition coil (cylinder 9); Damaged coil connector or harness at cylinder 9; Worn spark plug or carbon-tracked boot (cylinder 9); Clogged or leaking fuel injector (cylinder 9); Low compression on cylinder 9; Vacuum leak at the cylinder 9 intake runner.
What are the symptoms of a P0309 code?
Typical symptoms: Check engine light.
Is it safe to drive with a P0309 code?
Depends on the light. Flashing: stop as soon as it is safe to — raw fuel is reaching the catalytic converter. Steady, and it only feels rough: a short drive to a shop is usually fine. Either way, stop if it shakes hard, loses power, overheats or makes a new noise. Estimated repair: $50-$2,000.
How do I diagnose a P0309 code?
The ECU has already told you which cylinder. That makes the fastest honest test a swap: move one part to another cylinder and see whether the fault moves with it. It identifies the fault instead of guessing at it, and it costs nothing. 1. Photograph the coils before you touch anything, and note which is which. 2. Move the cylinder 9 ignition coil to a cylinder that is not misfiring, put that cylinder's coil on 9, clear the code, and run it until the misfire returns. 3. Read the plug you pulled, and check the boot for carbon tracking. 4. If spark is accounted for, go to fuelling, then to compression.
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