The check engine light: the complete guide

What the light actually means

The check engine light is the ECM’s way of saying "I detected something outside the expected range". Behind every light there is at least one diagnostic trouble code (DTC) — a five-character code the ECM stores.

• Steady light: usually emissions or a sensor — not urgent, but read the code soon
• Flashing light: a severe misfire that can destroy the catalytic converter — stop when safe
• Light off but code stored: an intermittent fault — check pending codes

What to do first

Before anything, check the simplest causes. A loose or missing fuel cap is behind a surprising number of lights and triggers EVAP codes like P0442 or P0455.

• Check and tighten the fuel cap
• Note if the car drives normally or feels wrong
• Read the code with any OBD-II scanner or the free AutoTools lookup
• Fix the confirmed cause before clearing the code

The most common codes decoded

These five codes cover a large share of real-world check engine lights. Use the full OBD-II lookup for the complete list.

• P0300–P0304: misfire — spark plugs, ignition coils, injectors, vacuum leaks
• P0171/P0174: too lean — vacuum leak, MAF, fuel pressure
• P0420/P0430: catalyst efficiency below threshold
• P0128: thermostat stuck open (engine runs cold)
• P0442/P0455: EVAP system leaks — fuel cap first

When it is safe to drive

A steady light with a normal-feeling car is generally safe for a short trip to a garage or to read the code. A flashing light, a shaking engine or a loss of power means stop and have it towed if possible.