What P0308 means
The engine computer has traced a misfire to cylinder 8, meaning that cylinder isn't burning its fuel properly. You'll usually feel shaking at idle, stumbling under load or less power. Only V8 and larger engines have a cylinder 8, and its coil, spark plug or their connections are the likely causes.
How the engine computer detects it
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. P0308 means more than 90 % of the detected misfires fall on cylinder 8. 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. On V8 engines GM bulletins name loose coil connectors and a loose coil ground on the cylinder head.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
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
Common causes
- Ignition coil of cylinder 8 faulty (coil, connector, ground) Most likely
- Spark plug of cylinder 8 worn, sooted, cracked or wrongly gapped Most likely
- Injector of cylinder 8 clogged or leaking, or its control Possible
- Low compression on cylinder 8 (valve, rings, head gasket) or local false air Less likely
- Water or oil in the plug well Less likely
How to diagnose P0308
- Locate cylinder 8. On most V8s it's at the back of one cylinder bank, close to the firewall; which side depends on the engine, so check the repair manual.
- With the ignition off, check the cylinder 8 coil connector and ground. Rear coils and connectors are easy to knock loose during other work behind the engine.
- Swap the cylinder 8 coil with another cylinder's, clear the code and drive. If the misfire follows the coil, replace it.
- Pull the cylinder 8 spark plug and check the gap, cracks and deposits. Oil in the plug well usually means a leaking valve cover gasket or tube seal.
- If other cylinders on the same bank also misfire, look for something they share, like an intake leak on that side.
- If coil and plug are fine, have the injector tested, then a compression or leak-down test done on cylinder 8.
Vehicles that report P0308 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| GMC Sierra 1500 | 2018–2019 | 3 | — |
Counts of owner complaints to the NHTSA that mention P0308 (12 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 3
- Ignition system or misfire
- 08
- Specific fault
Questions drivers ask
Can I drive with code P0308?
A flashing check engine light means stop as soon as it's safe: unburned fuel is overheating the catalytic converter. If the light is steady, you can drive gently a short distance to a shop, but don't tow or carry heavy loads until it's fixed.
Will my car pass an emissions inspection with P0308?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0308 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0308 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.
Often reported together
Codes that owners mention in the same NHTSA complaints as P0308. They can point to a shared cause, so it's worth checking them together.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.