What the check engine light means

Cars and light trucks sold in the U.S. since the 1996 model year carry an on-board diagnostics system known as OBD-II. The engine computer watches dozens of sensors, and when a reading falls outside its expected range, it stores a trouble code and may turn on the check engine light. The light covers a wide range of problems, from a loose gas cap to a misfire that can damage expensive parts. It can't tell you which one on its own.

Steady or flashing

Steady light. Something needs attention. It's usually safe to keep driving carefully until you can get the car looked at, unless the car is running badly, overheating or losing power.
Flashing light. Usually a serious misfire. Unburned fuel can overheat and damage the catalytic converter. Reduce speed and get the car checked as soon as you safely can.

If another warning light comes on with it, such as oil pressure or engine temperature, pull over safely and stop the engine.

A code is a clue, not a diagnosis

Parts stores will read codes for free, and inexpensive code readers are easy to buy. That's useful, but a code only says which system noticed a problem. A code for a lean condition, for example, can come from a vacuum leak, a dirty sensor, a weak fuel pump or a leaking injector. Replacing the part named in the code's description, without testing, is how people spend money and keep the light.

What a real diagnosis involves

Reading all stored codes, including pending ones, and the freeze-frame data that shows conditions when the code set.
Checking for known issues. Makers publish technical service bulletins about common problems on particular models.
Live data. Watching sensor readings while the engine runs, to see what's actually happening.
Pinpoint tests. Testing the suspect part and its wiring with a meter, a smoke machine for vacuum leaks, a fuel pressure gauge and similar tools.
Confirming the fix. After a repair, clearing the codes and driving the car under the conditions that set them, to make sure they stay off.

Diagnostic fees

Diagnosis takes a technician's time and equipment, and most shops charge for it. Some apply the fee toward the repair if you go ahead. Ask how the fee works, what it covers, and what happens if the first round of testing doesn't find the cause.

Common causes

loose or worn gas caps and other evaporative system leaks;
oxygen sensors and air-flow sensors;
ignition coils and spark plugs causing misfires;
vacuum leaks from cracked hoses or gaskets;
catalytic converter efficiency codes, which often have another cause upstream.

Emissions inspections

Some states and areas require emissions or smog inspections, and a lit check engine light can mean a failed test. Rules differ by state and county, so ask the shop what applies where your vehicle is registered.

Intermittent problems

Some faults only show up on cold mornings, in heavy rain or at highway speed. Write down when the light comes on and what the car was doing. That information can save diagnostic time.

Questions to ask

Which codes did you find, and what tests did you run?
What's causing the code, and how sure are you?
What does diagnosis cost, and is it applied to the repair?
Is it safe to drive in the meantime?
How will you confirm the repair fixed it?