P0351
Ignition Coil A Primary/Secondary Circuit MalfunctionP0351 is a generic OBD-II powertrain diagnostic trouble code: Ignition Coil A Primary/Secondary Circuit Malfunction. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0351 means
P0351 is stored when the ECM detects a fault in the primary or secondary circuit of Ignition Coil A — the coil assigned to cylinder 1 (or the first coil in a distributor-less waste-spark or coil-on-plug arrangement). The ECM monitors the ignition coil primary circuit by watching the feedback voltage collapse when the driver transistor fires; if the collapse is absent, too slow, or produces no corresponding secondary event, the code is stored. On coil-on-plug (COP) systems, coil A is directly above cylinder 1 and its failure will cause a hard, cylinder-specific misfire (typically accompanied by P0301).
Primary circuit faults include an open or shorted coil primary winding, a failed ignition module or PCM coil driver, and damaged wiring between the PCM and the coil. Secondary circuit faults include a cracked or tracking coil tower, a fouled or gap-eroded spark plug that demands excessive secondary voltage, or a damaged spark plug wire on older distributor-type systems. Because high secondary voltages (20–40 kV) can arc through microscopic cracks — especially in a humid or cold engine bay — secondary failures often appear as intermittent misfires before a hard code is set.
P0351 warrants prompt attention because prolonged misfiring on cylinder 1 dumps raw fuel into the exhaust stream, which can rapidly damage the catalytic converter through thermal overload.
Common causes
Most-frequently reported root causes when P0351 is logged.
-
1
Failed ignition coil A (open primary winding, cracked secondary tower, or internal short)
-
2
Faulty or fouled spark plug on cylinder 1 causing excessive secondary demand
-
3
Damaged primary wiring or connector between ECM and coil A (open, short to ground, short to B+)
-
4
Failed PCM/ECM coil driver circuit for coil A channel
-
5
Cracked spark plug boot or wire allowing secondary voltage to arc to ground (older systems)
-
6
Water or oil contamination inside the coil-on-plug well
-
7
Weak or failing ignition module (on distributor or remote-module DIS systems)
Symptoms drivers notice
How to diagnose P0351
A typical diagnostic flow when this code is present.
-
1
Scan all codes; confirm whether P0301 or other misfire codes accompany P0351 — this confirms coil A is failing rather than a different misfire source
-
2
On COP systems: swap the coil from cylinder 1 to a known-good cylinder, clear codes, and test drive — if the misfire and P035x code follow the coil to the new cylinder, the coil is faulty
-
3
Inspect the coil connector and wiring for corrosion, pushed-back pins, chafing, and oil/water contamination in the plug well
-
4
Measure coil primary resistance at the coil terminals (typical spec 0.5–2 Ω primary; 6–30 kΩ secondary depending on design); open or out-of-spec readings confirm failed coil
-
5
Verify PCM is supplying the correct trigger signal to coil A using a test light or oscilloscope — no trigger indicates a PCM driver fault
-
6
Inspect and replace the spark plug on cylinder 1 if fouled, gap-eroded, or cracked; a heavily fouled plug can kill an otherwise healthy coil
-
7
Check for oil intrusion into the COP well via the valve cover gasket, which degrades both the coil boot and coil body
Related powertrain codes
Frequently asked questions
Why do P0351 and P0301 appear together?
P0301 is a cylinder 1 misfire count code — it fires whenever that cylinder misfires above the ECM threshold. P0351 is the ignition circuit fault code that explains why. They are expected companion codes when the ignition coil is the misfire cause. If P0301 appears without P0351, look at fuel injector, compression, or cam timing issues instead.
Can a bad spark plug cause P0351?
Yes indirectly. A heavily fouled, cracked, or over-gapped spark plug demands a much higher secondary voltage to fire. This additional load can cause the primary collapse waveform to fall outside the ECM's expected window, triggering P0351 even though the coil itself is not defective. Always inspect and replace the cylinder 1 plug as part of P0351 diagnosis.
How do I know if it is the coil or the PCM driver that failed?
Swap the coil to another cylinder and monitor whether the misfire/fault code follows the coil. If the new cylinder now misfires and sets that cylinder's coil code (e.g. P0352), the coil is defective. If cylinder 1 continues to misfire with a known-good coil installed and P0351 returns, the PCM driver circuit for coil A is suspect.
Is it safe to drive with P0351 active?
Not recommended beyond a short distance to reach a workshop. Continuous misfiring on cylinder 1 sends unburned fuel into the catalytic converter, where it ignites and causes rapid thermal damage. Catalyst replacement is far more expensive than an ignition coil, so the code should be addressed the same day it appears.
Disabling P0351 in software
RaceTune can permanently disable P0351 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P0351 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
Upload your file