Clearing codes and readiness monitors

A scan tool can erase stored codes and switch off the check engine light. That is useful after a repair. It is not a repair in itself.

What clearing does

Clearing wipes the stored codes, the freeze frame data and the results of the computer's self-tests. The light goes out. If the fault is still there, the computer will find it again and the light returns, sometimes within minutes, sometimes after a few days.

Readiness monitors

The computer runs a set of self-tests called monitors: for the catalytic converter, the oxygen sensors, the fuel vapour system, the EGR system and others. After codes are cleared, or the battery is disconnected, every monitor shows "not ready" until it has run again.

In places where the emission inspection reads the car's computer, a car with monitors not ready is turned away. Clearing a code just before such a test does not help.

Drive cycles

Monitors run only under certain conditions: a cold start, a steady cruise, some idling, a tank that is neither full nor nearly empty. A mix of town and open-road driving over a few days normally completes them all.

Pending, stored and permanent codes

• Pending: the fault has been seen once. If it does not return, the code disappears by itself.
• Stored (or confirmed): the fault was seen on two trips and the light is on.
• Permanent: a copy of a confirmed emission code that cannot be erased with a tool. It clears only when the computer has re-tested the system and found it healthy.

Disconnecting the battery

Taking the battery lead off may clear codes on older cars, but it also wipes the radio code, clock, window settings and the engine's learnt idle and fuel values, and on modern cars it may not clear the code at all. It is a poor substitute for a scan tool.

When clearing makes sense

• After the cause has been repaired.
• After a one-off event you understand, such as a fuel cap left loose.
• As a test: clear, drive, and see which codes come back first.

Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.