GarageGuideIgnition coil circuit (coil A)
P0351 Code: Ignition Coil A Primary/Secondary Circuit
P0351 stands for "Ignition Coil A Primary/Secondary Circuit Malfunction." Modern engines usually have one ignition coil per cylinder. The engine computer switches each coil on and off to create the spark, and it watches the electrical circuit while doing it. P0351 means the computer saw an electrical problem with coil A's circuit.
On most engines coil A is the coil for cylinder 1, coil B is cylinder 2, and so on, but check this for your engine. P0351 is about the electrical side of the coil, while a misfire code like P0301 is about the cylinder not firing. They often appear together, because an electrically failed coil also causes that cylinder to misfire.
A failed coil means that cylinder is not firing reliably, so the engine runs rough and unburned fuel can reach the catalytic converter. If the check engine light is flashing, treat it as urgent.
If the engine runs reasonably and the light is steady, drive gently to get it fixed. If the light is flashing or the engine shakes badly, stop driving to avoid damaging the catalytic converter.
Symptoms you might notice
- Check engine light on, sometimes flashing
- Rough idle or shaking
- Loss of power and hesitation
- Misfire code for the matching cylinder, such as P0301
- Worse fuel economy
- Sometimes hard starting
Common causes
Failed ignition coil
The coil can short or open internally from heat and age. This is the most common cause.
Damaged connector or wiring
A cracked connector, broken locking tab, or chafed wire to the coil interrupts the circuit.
Oil or water in the spark plug well
Oil from a leaking valve cover gasket, or water from washing the engine, can damage the coil and connector.
Blown fuse or relay for coil power
On many cars, the coils share a power supply. If several coil codes appear at once, the shared power feed is a suspect.
Engine computer driver fault
Rarely, the circuit inside the computer that switches the coil fails. This is considered only after the coil and wiring check out.
Before you buy parts
P0351 points to the electrical circuit for one coil. The quick swap test usually tells you whether the coil or the wiring is at fault, using only basic tools. If several coil circuit codes appear at once, do not replace every coil; check the shared fuse, relay, and ground first.
What to check
- Read all codes. P0351 with P0301 strongly points to coil A. Several coil codes at once (P0351, P0352, P0353) point to shared power, a fuse, or a ground rather than several bad coils.
- Confirm which cylinder coil A is on your engine. On most engines it is cylinder 1.
- Inspect the coil connector for cracks, a broken locking tab, corrosion, or a loose fit. Wiggle it with the engine idling and listen for the idle to change.
- Swap test: with the engine off, swap coil A with a neighboring coil, clear the codes, and drive. If the code changes (for example to P0352), the coil is bad. If it stays P0351, look at the wiring.
- Pull the coil and check the spark plug well for oil or water.
Possible repairs
Replace the ignition coil
Once the swap test confirms it, replacing a single coil is usually a quick DIY job.
Repair the connector or wiring
Replacing a cracked connector or repairing a damaged wire fixes the circuit without a new coil.
Fix the oil leak into the plug well
Replacing the valve cover gasket and plug tube seals prevents the new coil from failing the same way.
Replace the fuse or relay
If several coils lost power at once, a fuse or relay may be all it needs.
Replace the spark plug while you are there
If the plugs are due, replacing them along with the coil saves doing the job twice.
Parts and tools that may help
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Frequently asked questions
What is the difference between P0351 and P0301?
P0351 means the computer saw an electrical problem with coil A's circuit. P0301 means cylinder 1 is misfiring. A failed coil A often causes both.
Which cylinder is coil A?
On most engines coil A is cylinder 1, coil B is cylinder 2, and so on. Confirm for your specific engine.
Can I drive with P0351?
Gently, if the light is steady and the engine runs reasonably. If the light is flashing, stop driving to protect the catalytic converter.
How do I test an ignition coil?
The easiest way is a swap test: move the coil to another cylinder and see if the code moves with it.
Should I replace all the coils?
Not usually. Replace the failed coil. If the coils are very old and you are replacing spark plugs anyway, some owners replace them all, but it is not required.