Does P0351 always mean the ignition coil is faulty?

No. The code describes a fault in the coil's electrical circuit, and that circuit includes the connector, the wiring back to the control unit, the supply, the earth and the driver inside the unit itself. Backed-out or corroded connector pins are very common, as are boots that have perished or filled with oil from a leaking cam cover gasket. Inspect the connector and the boot before ordering a coil.

Can I swap coils to test a P0351?

Yes, and it is the quickest test available, but read the result the right way round. Move the cylinder one coil to another cylinder, clear the codes and drive the car. If the fault code moves with the coil to its new cylinder, the coil is faulty. If P0351 comes back on cylinder one with a different coil fitted, the coil was never the problem and the fault lies in the wiring, the connector or the control unit output for that cylinder.

What is the difference between P0301 and P0351?

They describe different things measured in different ways. P0301 is a misfire code, worked out by measuring how much the crankshaft slows after cylinder one should have fired, so it tells you combustion failed without saying why. P0351 is an electrical code, worked out by monitoring current and voltage in the coil's primary circuit, so it tells you the circuit is faulty without reference to how the engine ran. They very often appear together, but each can appear alone: a weak spark from an electrically healthy coil gives P0301 with no coil code, and an intermittent wiring fault can give P0351 on an engine that never misfired badly enough to be counted.

Will a P0351 fail an MOT?

Yes. An illuminated engine malfunction indicator lamp is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, and the car fails on the lamp alone. If the coil fault is causing a real misfire, the raised hydrocarbon reading at the emissions test will usually fail it a second time.

Powertrain code · standard

P0351Ignition coil circuit fault, cylinder 1

Standard definition: Ignition Coil "A" Primary/Secondary Circuit Malfunction (Cylinder 1)

Last updated 27 July 2026

Quick answer

The electrical circuit for the ignition coil on cylinder one is not behaving as the control unit expects. Unlike a misfire code this is measured electrically, so it names a circuit rather than a symptom, and the fault is as often the connector or the wiring as the coil itself.

How the car works this out

The control unit charges the coil's primary winding and then switches it off to fire the plug, and it watches the electrical behaviour of that circuit each time. It can see how much current flows while the coil charges and whether the expected voltage spike appears when the current is interrupted. Too little current means an open circuit or a bad connection; too much means a short; no spike at all means the coil is not doing its job. None of that involves measuring whether combustion happened, which is the key difference from P0301. A coil can be electrically perfect and still produce a spark too weak to light the mixture, giving a misfire code with no coil code, and it can be electrically faulty on an engine that still runs, giving a coil code with no misfire code.

What you might notice

A misfire on cylinder one, often with P0301 stored alongside
A rough idle and a stumble under acceleration
An intermittent fault that comes and goes with heat, damp or road vibration
The engine management lamp on, flashing if the cylinder is dead under load
In some cases no driving symptoms at all, because the fault is intermittent and brief

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The coil itself, with an internal open circuit or a broken windingCommon
How to check: Swap the coil onto another cylinder. If the code moves to that cylinder's coil code, the coil is at fault. If P0351 stays where it was, the problem is in the wiring or the connector rather than the coil, which is the opposite conclusion to the same swap on a misfire code.
2The connector at the coil: a pin pushed back out of the housing, green corrosion on the terminals, or a locking tab never clicked home after previous workCommon
How to check: Unplug it and look inside with a torch. Terminals should be bright and springy; pins that sit lower than their neighbours have backed out. Wiggle the connector with the engine idling and see if anything changes.
3A perished, split or oil-soaked rubber boot letting the spark track to earth down the outside of the plug rather than jumping the gapCommon
How to check: Inspect the boot for cracks and carbon tracking, which shows as thin black lines on the rubber or on the plug's ceramic.
4Oil in the spark plug well from a leaking cam cover gasket, which destroys boots and eventually coils one after anotherPossible
5Chafed or broken wiring between the coil and the control unitPossible
6A spark plug with an eroded, oversized gap or a cracked insulator, which raises the voltage the coil has to produce until it flashes over internally and destroys itselfPossible
How to check: Check the plug gap against the specification. A plug well past its interval is a common reason a new coil fails soon after fitting.
7A failed coil driver output inside the control unitLess common

What to do about it

1Confirm which cylinder is number one on your engine. It is at the timing drive end of an inline engine on almost all of them, but on a V engine it defines which bank is bank one and the position varies by manufacturer.
2Unplug the coil and inspect the connector and the boot before ordering a coil. Backed-out pins and oil-soaked boots are extremely common and cost far less to put right.
3Swap the coil with a neighbouring cylinder, and read the result carefully. If the fault code follows the coil, replace the coil. If P0351 stays on cylinder one with a different coil fitted, the fault is in the harness, the connector or the control unit output.
4Fix any oil leak into the plug wells at the same time. Fitting a new coil into a well full of oil is a repair with a short shelf life.
5Replace the spark plug if it is anywhere near its interval. A worn plug is what kills coils, so leaving the old one in is how people end up buying two coils.

Can you keep driving?

Only to a garage

It will run, though probably badly, and a short trip to a garage is fine. The reason not to keep using it is what a dead cylinder does downstream: unburnt fuel arriving at a hot catalytic converter is the fastest way to destroy one. If the lamp is flashing rather than steady, that process is already under way and the car should be stopped.

Will it fail an MOT?

The lamp fails the car on its own — a major defect on petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and the largest warning-lamp defect in the DVSA data behind this site at 371,859 failures. What makes the coil codes worth catching early is the second-order damage: a cylinder that is not firing sends its fuel straight into the catalytic converter, and a car that arrives at its test with a ruined converter as well as a coil fault will also fail the carbon monoxide and hydrocarbon readings. The cheap repair prevents the dear one.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Does P0351 always mean the ignition coil is faulty?: No. The code describes a fault in the coil's electrical circuit, and that circuit includes the connector, the wiring back to the control unit, the supply, the earth and the driver inside the unit itself. Backed-out or corroded connector pins are very common, as are boots that have perished or filled with oil from a leaking cam cover gasket. Inspect the connector and the boot before ordering a coil.
Can I swap coils to test a P0351?: Yes, and it is the quickest test available, but read the result the right way round. Move the cylinder one coil to another cylinder, clear the codes and drive the car. If the fault code moves with the coil to its new cylinder, the coil is faulty. If P0351 comes back on cylinder one with a different coil fitted, the coil was never the problem and the fault lies in the wiring, the connector or the control unit output for that cylinder.
What is the difference between P0301 and P0351?: They describe different things measured in different ways. P0301 is a misfire code, worked out by measuring how much the crankshaft slows after cylinder one should have fired, so it tells you combustion failed without saying why. P0351 is an electrical code, worked out by monitoring current and voltage in the coil's primary circuit, so it tells you the circuit is faulty without reference to how the engine ran. They very often appear together, but each can appear alone: a weak spark from an electrically healthy coil gives P0301 with no coil code, and an intermittent wiring fault can give P0351 on an engine that never misfired badly enough to be counted.
Will a P0351 fail an MOT?: Yes. An illuminated engine malfunction indicator lamp is a major defect on petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, and the car fails on the lamp alone. If the coil fault is causing a real misfire, the raised hydrocarbon reading at the emissions test will usually fail it a second time.

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