P0307
PowertrainCylinder 7 Misfire Detected
Cylinder 7 is misfiring, meaning it isn't burning its fuel and air mixture properly on each combustion stroke. Because cylinder 7 only exists on bigger engines, you're almost certainly looking at a V8 or similar, so think Range Rover, big BMW and Mercedes petrols, or American V8s like Mustangs and pickups. The job is to work out whether it's an ignition fault, a fuel fault, or something mechanical inside that cylinder, and the cost between those three is night and day.
ⓘ Information only. This page provides general educational information about fault code P0307. 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 P0307 mean?
P0307 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 7 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's flashing you've got a bad misfire that's dumping raw fuel into the exhaust
- • Lumpy, uneven idle that you can feel through the seat and the steering wheel
- • Hesitation or a flat spot when you put your foot down, particularly pulling away
- • Loss of power higher up the rev range when the engine is asked to work harder
- • Fuel economy drops off because that cylinder is wasting its fuel
- • On a really bad misfire, the car may drop into limp mode to protect the cat
Possible causes
Causes commonly associated with P0307, 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 in cylinder 7. Cheapest and most common cause, and on a V8 these plugs can be a pig to reach so they get neglected
- 2. Failed or weak ignition coil on cylinder 7. Coil-on-plug failures are very common on BMW and Mercedes V8s, and the coil usually only powers that one cylinder
- 3. Faulty or partially blocked fuel injector for cylinder 7, so the cylinder is starved of fuel or getting an uneven spray pattern
- 4. Damaged plug lead on older engines that still run separate HT leads to each plug
- 5. Vacuum leak near that side of the intake leaning out the mixture on cylinder 7
- 6. Low compression from worn rings, a burnt valve, or a head gasket fault, the expensive end and usually accompanied by oil burning or coolant loss
- 7. ECM fault or wiring problem in the cylinder 7 circuit. Rare, only worth chasing once the obvious stuff is ruled out
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 codes and read the freeze frame data. Note whether you've also got P0300 or misfires on other cylinders, because a single P0307 points at one cylinder while a scatter of codes points at fuel pressure or a wider fault
- 2. Swap the cylinder 7 ignition coil with one from a known-good cylinder, clear the code, and drive it. If the misfire jumps to the cylinder you moved the coil to, you've found a dud coil
- 3. Pull and inspect the cylinder 7 spark plug. Look for oil fouling, a worn or wide gap, or a cracked insulator, and compare it against the plugs from the other cylinders
- 4. Check the cylinder 7 wiring, connector, and harness for green corrosion, chafing, or a loose pin before you condemn anything expensive
- 5. If ignition checks out, swap or test the injector for that cylinder. A blocked injector or one with a poor spray pattern will misfire under load
- 6. Last resort is a compression test on cylinder 7 against the rest. Low numbers here mean the fault is mechanical and you're into proper engine work
Common questions about P0307
How do I know if it's the coil itself or just the wiring to it? +
Easiest way is the swap test. Move the cylinder 7 coil to another cylinder, clear the code, and see if the misfire follows the coil. If it does, the coil is the fault. If the misfire stays put on cylinder 7 with a known-good coil fitted, the problem is upstream, so check the connector and the wiring back to the ECM for corrosion or a broken pin. A multimeter on the coil connector with the engine cranking will tell you whether it's getting power and a trigger signal. Most P0307s on V8s turn out to be the coil rather than the loom.
How long does this take to sort out? +
If it's a coil or a plug and you can get to cylinder 7 easily, a garage will have it done in under an hour. The trouble on a lot of V8s is access. Mercedes and BMW V8s often need the intake manifold off to reach the back cylinders, which can turn a 20 minute job into a couple of hours of labour. An injector is similar, mostly access time. A compression test plus the engine repair it leads to is a different world, that's a day or more and proper money.
Should I fit a cheap aftermarket coil or pay for genuine? +
For coils, stick to a recognised brand like Bosch, NGK, Delphi, or genuine. The bargain eBay coils are a false economy and tend to fail again within months, which on a buried V8 cylinder means doing the whole job twice. Spark plugs should match the exact spec the engine wants, so use the manufacturer's recommended part and don't substitute a cheaper grade. Injectors are usually best bought genuine or properly remanufactured rather than no-name parts.
Can I keep driving it for now? +
Short hops to the garage are fine if the light is steady, though the car will feel rough and drink fuel. If that warning light is flashing, stop. A flashing light means a heavy misfire that's pumping unburnt fuel into the exhaust, and that will cook the catalytic converter, which is far dearer to replace than a coil. Pull over, switch off, and get it recovered rather than risk turning a cheap fault into an expensive one.
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 →