Powertrain, generic · Ignition system or misfire

P0309 Cylinder 9 Misfire Detected

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

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

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

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–800
Time0.3–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ignition coil cylinder 9spark plug cylinder 9fuel injector cylinder 9

Related codes

P0300P0359P0209P0316P0420

References: law.cornell.edu, newtonnet.co.uk, fordservicecontent.com, static.nhtsa.gov.