P0308
PowertrainCylinder 8 Misfire Detected
Usually a small job. Cylinder 8 isn't burning its fuel properly, and most of the time it comes down to a tired coil or spark plug that's cheap to put right. The ECU spots cylinder 8 spinning slightly slower than the others through the crank sensor and flags it as a misfire. Because P0308 only shows on 8-cylinder engines, you're looking at the likes of older Range Rovers, Jaguars, some BMW and Mercedes V8s, and a few American imports, so coil-on-plug is the usual setup and the failed part is normally easy to pinpoint.
ⓘ Information only. This page provides general educational information about fault code P0308. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0308 mean?
P0308 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 8 Misfire Detected.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, and if it flashes you've got a heavy misfire that's dumping raw fuel into the cat
- • Lumpy, shaky idle that you can feel through the steering wheel and seat
- • Clear flat spot or hesitation when you put your foot down
- • Petrol smell from the back of the car, strongest at idle or in slow traffic
- • Fuel economy drops off, the engine's effectively running on seven cylinders
- • On a bad misfire the engine may drop into limp mode to protect the cat
Possible causes
Causes commonly associated with P0308, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn or fouled spark plug on cylinder 8, the cheapest and most common culprit, especially if the plugs are overdue a change
- 2. Failed ignition coil for cylinder 8. Coil-on-plug units cook over time and one usually goes before the rest
- 3. Cracked or perished plug boot letting the spark track to earth, common on the V8s where boots sit in deep wells and bake
- 4. Faulty or partially blocked injector on cylinder 8, less common but it does happen on direct-injection engines
- 5. Wiring or connector fault at the coil or injector, look for chafed looms and green corrosion on the pins
- 6. Vacuum leak near cylinder 8 leaning out the mixture for that one pot
- 7. Low compression from a burnt valve, tired rings, or a head gasket starting to let go, this is the expensive end and the last thing you check
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull the live data and the freeze frame so you can see whether the misfire happens cold, hot, at idle, or under load. That alone narrows it down fast
- 2. Check the misfire counter per cylinder if your tool reads it. A count climbing only on cylinder 8 confirms it's isolated and not a general P0300
- 3. Swap the coil from cylinder 8 onto a known-good cylinder, then clear the code and drive it. If the misfire moves with the coil, you've found it. Do the same with the plug if the coil checks out
- 4. Look closely at the plug, boot, and connector on cylinder 8 for cracks, oil, soot, or corrosion before you spend on parts
- 5. Check the injector is firing and the wiring is clean if ignition swaps come up empty
- 6. Run a compression or leak-down test on cylinder 8 last, once ignition and fuel are ruled out. Low compression means the problem is inside the engine and the bill changes completely
Common questions about P0308
What am I likely to pay to sort this out? +
If it's a single coil or plug, you're looking at roughly £40 to £120 in parts and well under an hour's labour, so most independents will have you out the door for under £150. A set of plugs for a V8 runs more because there are eight of them. An injector is dearer, maybe £150 to £400 fitted at an independent. Where it hurts is internal engine work, a burnt valve or head gasket job on these V8s can run well into four figures. Main dealers will charge noticeably more on labour, so for a straightforward coil swap a good independent is the sensible call.
How do I work out which of these it actually is on my car? +
The swap test does most of the work for you. Move the cylinder 8 coil to another cylinder and clear the code. If the misfire follows the coil, replace the coil. If it stays on cylinder 8, do the same with the plug. If the misfire still won't budge after both, you're into injector, wiring, or compression territory, and a compression test on that one cylinder tells you whether the trouble is mechanical. A misfire that only shows under load often points to ignition breaking down, while one that's there from cold start tends to be fuel or compression.
Can I just do the spark plug and coil myself? +
Yes, on most of these V8s the coil sits right on top of the plug and lifts off after one bolt or a clip, so it's an easy job with basic tools. Buy a decent coil and plug, don't chase the cheapest eBay listing because a weak no-name coil can throw the same code again in months. Get the spark plug gap and torque right when you fit it. Just don't blindly throw parts at it without the swap test first, or you'll spend money replacing things that were fine all along.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →