What the check engine light is
Cars and light trucks sold in the US since the mid 1990s use a standard onboard diagnostic system called OBD-II. It monitors the engine, fuel system, ignition and emissions equipment. When it sees a reading outside normal limits, it stores a diagnostic trouble code and turns on the check engine light, which is sometimes shaped like an engine outline or labeled "Service Engine Soon."
The light itself does not tell you what is wrong. It tells you a code is stored and something needs attention.
Steady light vs flashing light
A steady check engine light means a fault was detected but the system does not see an immediate danger. You can usually drive carefully to get it checked, but do not ignore it. Small problems can turn into bigger, more expensive ones, and the vehicle may fail an emissions inspection while the light is on.
A flashing check engine light usually means an active, severe engine misfire. Unburned fuel can overheat and damage the catalytic converter. Slow down, avoid hard acceleration and heavy loads such as towing, and stop driving as soon as it is safe. Have it looked at before driving further.
Common causes
There are many possible causes, from minor to serious. Some of the more common ones:
- A loose, cracked or missing gas cap
- Worn spark plugs or failing ignition coils
- A faulty oxygen sensor
- A mass airflow sensor problem
- Vacuum leaks from cracked hoses
- Evaporative emissions system leaks
- Catalytic converter problems
- Thermostat or engine temperature sensor faults
What you can check yourself
Start with the gas cap. Make sure it is present, not cracked and tightened until it clicks. If the cap was the cause, the light may not go off right away; the system needs to recheck itself over some normal driving.
Notice how the vehicle feels. Rough idle, shaking, hesitation, loss of power, a rotten egg smell, overheating or unusual noises are all worth writing down. Take a photo of the dashboard showing the light and any other warnings. These details help a mechanic find the problem faster.
When to stop driving
Pull over safely and turn off the engine if any of these happen, and do not keep driving to reach a shop. A mobile mechanic can come to the vehicle instead.
- The check engine light is flashing
- The red oil pressure warning comes on
- The temperature gauge is in the red or steam is coming from under the hood
- You smell fuel or see smoke
- The vehicle loses significant power or makes loud knocking noises
How a mechanic diagnoses it
A mechanic plugs a scan tool into the diagnostic port, usually under the dashboard on the driver side, and reads the stored codes and live data. The code is a starting point. For example, a misfire code tells you which cylinder is misfiring but not whether the cause is a plug, a coil, an injector or something mechanical. Good diagnosis means testing before replacing parts.
Free code reads can be helpful, but a code alone often leads to guessing. Clearing the code without a repair only turns the light off until the fault returns. Ask whether there is a diagnostic fee and whether it applies toward the repair.
Getting it checked at home
Check engine diagnostics are one of the most common mobile mechanic jobs, because the mechanic can scan, test and often repair on the spot. With Trackara Find, you can send a free request describing the light and any symptoms to mobile mechanics near you. Learn more about check engine light diagnostics or read how to describe a car problem.