What turns on the check engine light?

The check engine light is turned on by the powertrain control module (PCM). The PCM monitors the engine's performance and emissions, and if it detects a problem, it will turn on the check engine light. The light may also be turned on if the PCM detects a problem with the transmission, fuel system, or emissions system.

Here are some of the things that can cause the check engine light to come on:

- A loose or disconnected gas cap.

- A bad oxygen sensor.

- A bad catalytic converter.

- A bad spark plug or spark plug wire.

- A bad ignition coil.

- A bad fuel injector.

- A bad fuel pump.

- A vacuum leak.

- An exhaust leak.

- A transmission problem.

If the check engine light comes on, it's important to have the vehicle diagnosed as soon as possible. Driving with the check engine light on can damage the engine or transmission.