Ignition coil codes

How its codes differ

• A cylinder misfire code (P0301 to P0306) reports that one cylinder is not firing; a weak coil is one of several causes and the coil swap test separates it from the plug and injector.
• P0300 (random or multiple misfire) is less likely to be one coil, unless one cylinder's misfire is bad enough to upset the others.
• P0351 is the electrical circuit for ignition coil A: it reports a wiring or driver problem in the coil's control rather than a misfire result.
• Converter codes (P0420, P0430) can follow misfire from a weak coil, because unburned fuel overheats the converter.

Codes

• P0300

  Random or multiple-cylinder misfire detected

  Severity: Inspect soon

• P0301

  Misfire detected in cylinder 1

  Severity: Inspect soon

• P0302

  Misfire detected in cylinder 2

  Severity: Inspect soon

• P0303

  Misfire detected in cylinder 3

  Severity: Inspect soon

• P0304

  Misfire detected in cylinder 4

  Severity: Inspect soon

• P0305

  Misfire detected in cylinder 5

  Severity: Inspect soon

• P0306

  Misfire detected in cylinder 6

  Severity: Inspect soon

• P0351

  Ignition coil A primary or secondary circuit fault

  Severity: Inspect soon

• P0420

  Catalytic converter efficiency below the computer's limit (Bank 1)

  Severity: Monitor

• P0430

  Catalytic converter efficiency below the computer's limit (Bank 2)

  Severity: Monitor

Signs it's failing

• Check engine light, steady or flashing
• Shaking or rough running, often worse under load
• Loss of power
• Misfire code naming one cylinder

How to test it

1. With the engine off and cool, swap the coil from the misfiring cylinder with a neighbouring one, bolt both down, clear codes and drive. If the misfire moves to the other cylinder, the coil is the cause. (Use caution)

   Warning: Ignition systems produce very high voltage. Never touch coils, boots or plug wires with the engine running; swap parts only with the engine off and the key out.

2. If the misfire did not move, swap the spark plug the same way; a worn plug gap overloads the coil and can cause misfire on its own. (Use caution)

   Warning: Let the engine cool fully before removing plugs to avoid burns and thread damage.

3. For P0351 or a coil that fails repeatedly, have the coil circuit tested with service information. (Best left to a shop)

   Warning: Ignition circuits carry high voltage; leave circuit testing to a technician.

After the repair

• Replace worn spark plugs along with a failed coil if they are due, so the new coil is not overloaded.
• Clear codes and check misfire counters on a scan tool after a drive.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.