The engine module detects misfire from crankshaft speed: a cylinder that does not fire gives no acceleration impulse. At a catalyst-damaging rate (CARB: at least 5 % per 200 revolutions) the warning light flashes; at an emissions-relevant rate (at least 1 % per 1,000 revolutions – GM about 2–5 %, Ford 0.9–1.5 %) it comes on after two trips. Ford and GM may shut off a misfiring cylinder's injector to protect the catalyst. P0309 means more than 90 % of the detected misfires fall on cylinder 9. Usual causes are the ignition coil or spark plug of that cylinder, its injector, or low compression; a quick test is to swap coil or plug with a neighbouring cylinder and see whether the misfire moves. Cylinder 9 exists on ten-cylinder engines such as Ford's 6.8 litre V10; there an under-torqued crankshaft pulley bolt and engine software caused false misfire codes on all cylinders.
High severity. Can I keep driving?Avoid driving if the light is flashing or the engine is shaking: unburned gasoline can damage the catalytic converter.
Cylinder 9 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light on, flashing at a catalyst-damaging rate
- Rough idle, jerking, vibration
- Loss of power, hesitation
- Smell of unburnt fuel, higher consumption
Most likely causes
- Ignition coil of cylinder 9 faulty (coil, connector, ground)
- Spark plug of cylinder 9 worn, sooted, cracked or wrongly gapped
- Injector of cylinder 9 clogged or leaking, or its control
- Low compression on cylinder 9 (valve, rings, head gasket) or local false air
- Water or oil in the plug well
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
- It can trigger limp mode.
- It affects emissions.
What each character of P0309 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 3: Ignition and misfires
- 09: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0309?
Avoid driving if the light is flashing or the engine is shaking: unburned gasoline can damage the catalytic converter.
What causes code P0309?
The most common causes are: ignition coil of cylinder 9 faulty (coil, connector, ground); spark plug of cylinder 9 worn, sooted, cracked or wrongly gapped; injector of cylinder 9 clogged or leaking, or its control.
Related codes
- P0300Random/Multiple Cylinder Misfire Detected
- P0359Ignition Coil I Primary Control Circuit/Open
- P0209Cylinder 9 Injector A Circuit/Open
- P0316Engine Misfire Detected on Startup (First 1000 Revolutions)
- P0420Catalyst System Efficiency Below Threshold Bank 1
Sources
- law.cornell.edu: 13 CCR 1968.2
- newtonnet.co.uk: fordobd2012
- fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
- static.nhtsa.gov: SB 10095824 2532
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.