Check Engine Light On – Causes, Symptoms and How to Fix It
A Check Engine Light (CEL) means your car’s engine control unit has detected a fault and stored one or more OBD2 fault codes. Some issues are minor, while others can cause serious engine damage if ignored. This guide explains the most common causes, symptoms and how to diagnose the problem quickly.
Symptoms When the Check Engine Light Is On
- Engine runs rough or shakes
- Poor acceleration or hesitation
- Increased fuel consumption
- Strong fuel smell or sulfur smell
- No symptoms at all (common with EVAP faults)
Common Causes of a Check Engine Light
- Loose or damaged fuel cap – very common and easy to fix
- Faulty oxygen (O2) sensor – affects mixture and emissions
- MAF sensor issues – causes hesitation and incorrect fuel trims
- Ignition coil or spark plug failure – leads to misfires
- EVAP system leaks – small leaks trigger CEL without symptoms
- Catalytic converter problems – often caused by long-term misfires
Solid vs. Flashing Check Engine Light
- Solid light: Usually safe to drive short distances, but diagnose soon.
- Flashing light: Indicates active misfires. Stop driving as soon as possible to avoid catalytic converter damage.
How to Diagnose a Check Engine Light
The fastest way to find the cause is to read the stored fault codes with an OBD2 scanner. These tools show the exact code (e.g. P0171, P0300) and live data to confirm the issue.
- Connect an OBD2 scanner to the OBD port.
- Turn ignition ON.
- Read stored and pending fault codes.
- Check live data (fuel trims, O2 sensors, MAF, misfire counters).
- Repair the issue and clear the codes.
Recommended Tools for Diagnosing a Check Engine Light
- iCarsoft V2.0 (single-brand): Full-system diagnostics for one manufacturer.
- iCarsoft CR PRO: Multi-brand full-system scanner for households with several cars.
- OBDEleven Pro (VAG): Best choice for VW, Audi, Skoda and Seat owners.
When You Should Not Drive
- Check Engine Light is flashing
- Engine is shaking or misfiring
- Very low power
- Strong fuel smell or sulfur smell
- Smoke from engine bay or exhaust