Short answer: A steady check engine light means the engine computer has stored a fault and the car should be checked soon. A flashing light usually means a severe misfire that can damage the catalytic converter, so drive gently and get the car to a workshop straight away.
Steady light
The engine management system has found a fault and stored a code. The car may feel normal. It is usually safe to drive to a workshop, but do not ignore it for weeks: small faults such as a failing sensor can affect fuel use and emissions, and can hide a second fault that appears later.
Flashing light
A flashing light typically means the engine is misfiring. Unburnt fuel can overheat and damage the catalytic converter, which is an expensive part. Ease off, avoid hard acceleration, and get the car checked as soon as possible.
Common causes
Ignition coils and spark plugs, oxygen sensors, mass air flow sensors, vacuum or intake leaks, faulty thermostats, EGR and emissions faults, and on some cars a loose fuel cap. On turbocharged engines, boost leaks are a frequent cause too.
Why the code is not the answer
A fault code says which reading was out of range, not which part failed. A "lean mixture" code, for example, can come from an air leak, a sensor, a weak fuel pump or a clogged injector. Testing the suspect parts is what separates a correct repair from replacing parts on a guess.
What to tell your mechanic
When the light came on, whether it is steady or flashing, any change in how the car drives, sounds or smells, and whether it happened after refuelling or any recent work. These details save diagnosis time.
