Spark plug codes

How its codes differ

• A cylinder misfire code (P0301 to P0306) can come from a worn, fouled or damaged plug on that cylinder; swapping the plug to another cylinder and seeing whether the misfire follows separates it from the coil and injector.
• P0300 with several cylinders misfiring can come from plugs that are all worn or fouled, but shared causes such as a lean mixture should also be checked.
• Misfire from a bad plug can damage the converter, so converter codes (P0420, P0430) may follow a long-ignored plug problem.
• P0351 (coil A circuit) can follow a worn plug or oil or water in the plug well that has damaged the coil, so check the plug when a coil is replaced.

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

• 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

• P0351

  Ignition coil A primary or secondary circuit fault

  Severity: Inspect soon

Signs it's failing

• Misfire, shaking or rough idle
• Check engine light, steady or flashing
• Hard starting
• Loss of power

How to test it

1. With the engine off and fully cool, remove the plug from the misfiring cylinder and compare it with the others: look for wet, oily or carbon fouling, cracked insulators or a worn gap. (Use caution)

   Warning: Let the engine cool fully before removing plugs to avoid burns and damaged threads; never touch ignition parts with the engine running.

2. Swap the suspect plug with one from another cylinder, clear codes and drive. If the misfire moves, the plug is the cause. (Use caution)

   Warning: Work with the engine off and cool; torque plugs to the vehicle's specification.

3. If a plug is fouled, find out why (oil use, rich running, short trips), because a new plug can foul again. (Low risk)

After the repair

• Clear codes and check misfire counters after a drive.
• Fix the cause of any fouling, or the new plugs may foul.
• 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.