P0351
PowertrainIgnition Coil"A" Primary/Secondary Circuit
Usually a small, cheap job. P0351 points at the ignition coil on cylinder 1, or its wiring and connector, and on most cars the fix is a single coil swap for under £50 in parts. The ECU watches the primary and secondary circuit of that coil, and when the electrical signal goes wrong it logs the fault and often kills the spark to that cylinder. The thing to sort quickly is the misfire that comes with it, because raw fuel hitting a hot cat gets expensive fast.
ⓘ Information only. This page provides general educational information about fault code P0351. 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 P0351 mean?
P0351 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Ignition Coil"A" Primary/Secondary Circuit.
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, frequently sitting next to a P0301 misfire code
- • Lumpy idle, sometimes with a vibration you can feel through the seat or steering wheel
- • Flat spot or hesitation when you accelerate, worst at low revs
- • Misfire you can hear and feel, more obvious under load or pulling away from a junction
- • Fuel economy creeping up as the ECU tries to compensate
- • Hard starting, particularly first thing on a cold or damp morning
Possible causes
Causes commonly associated with P0351, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The coil itself has failed on cylinder 1. This is where most P0351 jobs end up, especially on coil-on-plug setups that run hot
- 2. Corroded or chafed wiring in the loom between the coil and ECU, common on older cars where the harness runs near hot or moving parts
- 3. A loose or perished connector at the coil. Worth checking before you spend a penny, the locking tab cracks and the pins back out
- 4. Worn or fouled plug on cylinder 1 making the coil work harder and stressing the circuit
- 5. Vacuum leak upsetting cylinder 1 enough to trip misfire detection alongside the coil code
- 6. Blown fuse or dodgy relay feeding the coil power supply
- 7. Failed coil driver inside the ECU. Rare, and the last thing you should suspect, not the first
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 all the stored codes and check live data to confirm cylinder 1 is the one playing up and whether a misfire code came with it
- 2. Have a proper look at the coil and its plug. Cracked casing, green corrosion on the pins, oil sitting in the plug well, any of those tells you a lot
- 3. Swap the cylinder 1 coil onto another cylinder, clear the codes and drive it. If the fault follows the coil to its new home, the coil is your culprit
- 4. Take the cylinder 1 plug out and read it. Worn electrodes, sooty fouling or a wrong gap can be the real reason the coil is struggling
- 5. Check the harness for continuity and a proper supply voltage to the coil with a multimeter, and wiggle-test the loom while you watch the readings
- 6. Smoke test or vacuum gauge for an intake leak around cylinder 1 if everything electrical checks out clean
Common questions about P0351
What happens if I just leave it and keep driving? +
Cylinder 1 keeps dumping unburnt fuel into the exhaust, and that fuel burns off inside your catalytic converter instead of the engine. Cats hate that. A coil that costs £30 to £50 today can cook a cat that runs £400 to £1,500 to replace if you leave the misfire going for weeks. The car may also drop into limp mode to protect itself, so you lose power on the motorway exactly when you need it. Short hop to the garage is fine. Months of commuting on it is asking for a bill.
How quickly do I actually need to deal with this? +
Treat it as days, not weeks. It won't strand you on the hard shoulder tomorrow, but every mile on a steady misfire is wearing on the cat and washing fuel past the rings. If the misfire is heavy and the light is flashing, that flashing light means active damage is happening, so park it and sort the coil before driving further. A steady non-flashing light gives you a bit more breathing room to book it in.
Is it the coil itself or the wiring causing this? +
On most cars it's the coil, and the swap test settles the argument in ten minutes. Move the suspect coil to a different cylinder, clear the code, drive, and see if the fault travels with it. If the code follows the coil, fit a new coil and you're done. If P0351 stays stubbornly on cylinder 1 after the swap, you're looking at the connector, the loom, or a power supply problem at that cylinder, and that's where the multimeter comes out.
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 →