OBD-II Code P0351 — What It Means & How to Fix It

What Does This Error Mean?

P0351 means the PCM detected a fault in the primary circuit of ignition coil A. Coil A typically refers to the coil serving cylinder 1 (but this varies by manufacturer). The primary circuit is the low-voltage side that the PCM controls directly. A fault here means the coil may not fire at all, causing a cylinder 1 misfire.

Affected Models

All vehicles 1996+
Common in Ford F-150 with coil-on-plug ignition
Common in Chevrolet and GMC trucks and SUVs
Common in Dodge and Chrysler V6 and V8 engines
Common in Jeep vehicles with Chrysler engines

Common Causes

Failed ignition coil on the coil A circuit with an open or shorted primary winding
Damaged or corroded wiring connector at the coil A ignition coil
Broken wire in the primary circuit between the PCM and the coil
Faulty PCM ignition driver circuit for coil A (rare)
Spark plug failure causing excessive load that damages the coil

How to Fix It

Locate coil A on your engine. On most four-cylinder and V6 engines with coil-on-plug ignition, coil A is the one sitting on top of cylinder 1. Inspect the wiring connector for corrosion, damage, or loose fit.

Some manufacturers label coils by position or bank rather than cylinder number. Check your service manual if coil location is unclear.
Test the coil primary resistance with a multimeter set to ohms. Probe the two primary terminals on the coil. Compare to manufacturer spec (typically 0.4-2.0 ohms for primary winding). Out-of-range reading means the coil has failed.

Also test secondary resistance between the primary terminal and the spark plug boot. This checks the whole coil in one step.
Swap coil A with a coil from another cylinder to confirm whether the coil itself is faulty. Clear the code and drive. If the fault code moves to the new cylinder location, the coil is bad. If it stays on coil A's circuit, check the wiring.

This swap test is the most reliable confirmation method before buying a replacement part.
Inspect and replace the spark plug for the cylinder served by coil A. A severely fouled or worn plug can damage a coil by creating excessive resistance in the secondary circuit. Replace plug and coil together for best results.

Installing a new coil on a worn plug wastes money — the plug can damage the new coil in the same way.
Check the wiring harness from the coil connector back toward the PCM. Look for broken wires, connector pins that have backed out, or insulation damage near exhaust heat. Repair before clearing the code.

Use proper solder-and-shrink connectors for any wiring repairs — avoid using just electrical tape on ignition circuit repairs.

When to Call a Professional

If replacing the coil and spark plug doesn't resolve the code, have a mechanic check the PCM driver circuit. A technician can scope the coil primary signal to confirm the PCM is sending a proper trigger. PCM failures are rare but do occur, especially on high-mileage vehicles. Diagnosis typically costs $80-$130. Coil replacement runs $30-$100 for the part; labor varies by access.

Frequently Asked Questions

What does 'primary circuit' mean?

A coil has two circuits — primary and secondary. The primary circuit is the low-voltage (12V) side that the PCM switches on and off. When the PCM triggers the primary circuit, it creates a high-voltage spark in the secondary circuit. P0351 means a problem is detected on that primary low-voltage side.

Will P0351 cause a misfire?

Almost always, yes. If the primary circuit is faulty, the coil may not fire at all. No spark means no ignition — that cylinder misfires. You may see both P0351 and P0301 (cylinder 1 misfire) at the same time. Fix the coil circuit first.

Can a bad coil damage my engine?

The coil itself won't damage the engine. But the resulting misfire can. Unburned fuel entering the exhaust overheats and damages the catalytic converter. Long-term misfiring can also wash cylinder walls with fuel, diluting oil and accelerating wear. Fix it promptly.