What to do when the check engine light comes on

The check engine light comes from the car's on-board diagnostics computer, which watches the engine and the emissions system. When it finds a problem, it stores a trouble code and turns on the light. The cause can be as minor as a loose gas cap or as serious as an engine misfire, and the first few minutes are for working out how urgent it is.

What you need

• A safe place to pull over
• A notes app or a pen
• The owner's manual, which shows the light and what it means for your car

Steps

1. 1

   Notice whether the light is steady or flashing

   A flashing light means a severe problem, such as a misfire, that needs attention quickly. A steady light means the car may not be running properly, even if it feels normal to drive.

   1 min

2. 2

   If it's flashing, ease off

   Keep driving to a minimum until a shop has looked at the car, and avoid high speeds and heavy loads such as towing.

   2 min

3. 3

   Check the gas cap

   Once you've stopped, check that the gas cap is there and tight. A loose or missing cap is a common cause. Refueling with the engine running can also turn the light on.

   2 min

4. 4

   Write down what the car is doing

   Note any new noise, smell, shaking, loss of power or other warning light, and when the check engine light came on. These details help the mechanic along with the code.

   3 min

5. 5

   Book a mechanic

   Have a mechanic look at the car even if the light is steady. The problem behind it can waste fuel, add pollution and lead to costlier repairs the longer it waits. The mechanic reads the code with a scan tool, makes the repair and scans the car again to confirm it.

   2 min

6. 6

   Check the light after the repair

   Start the car once the work is done. The light should come on briefly at start-up, then stay off.

   1 min

When the light goes off on its own

If the problem doesn't come back over the next few trips, the computer turns the light off. That doesn't mean the system is faulty, and the car needs nothing more unless the light comes back.

Don't hide the light

Don't clear the codes with a scanner or disconnect the battery to make the light go away. It doesn't fix the problem. On cars from the 2012 model year on, the computer also keeps a permanent record of the fault that neither a scanner nor a disconnected battery can erase. That record clears only after the computer checks again and finds the fault gone.

Steady lights and emissions tests

If your car has an emissions test coming up, get the repair done first. Clearing the codes after a repair restarts the computer's self-checks, so drive a gasoline car for about a week before the test. Getting ready for an emissions test covers the rest.