Check engine light and diagnostics

Finding the cause of a warning light, rough running or a fault that comes and goes, before any parts are replaced.

What this covers

A check engine light, a rough idle or a car that hesitates can each have several causes. Diagnosis means finding which one before parts are replaced.

If your car is registered in one of the 13 metro Atlanta counties where Georgia requires emissions testing, a lit check engine light means it fails the test. Georgia’s Clean Air Force advises having the light checked as soon as possible.

How the problem is found

A fault code names the system that reported a problem, not the failed part. A misfire code, for example, says a cylinder misfired but not why.

The code is the starting point. The likely causes are then tested one by one before a repair is recommended. Ask for the findings and an estimate before you approve the work.

If the light comes and goes, note when it happens. Faults that only appear cold, hot or at highway speed are easier to find with that detail.

Questions

My check engine light is on. What should I do?

Have it checked soon. If the light is flashing, slow down and have it checked right away. Georgia’s Clean Air Force notes a loose fuel cap can turn the light on: tighten it until it clicks and see if the light goes off after some driving. In the 13 metro Atlanta emissions counties, a car with the light on fails the emissions test.