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.

Step-by-Step Diagnosis

1. Check whether the light is solid or flashing.
   A flashing Check Engine Light can indicate an active misfire and should be
   treated as urgent. If the light is solid and the car runs normally, continue
   with the checks below.
2. Check for obvious problems.
   Make sure the fuel cap is properly tightened and look for visible issues such
   as disconnected hoses, damaged wiring or other obvious faults in the engine bay.
3. Read the stored fault codes.
   Connect an OBD2 scanner and scan the vehicle for diagnostic trouble codes (DTCs).
   Write down the codes before clearing anything, as they provide the starting point
   for diagnosis.
4. Look up the fault code and diagnose the cause.
   A fault code identifies the system or condition that triggered the warning,
   but it does not always identify the failed component. Use our
   OBD2 Fault Code Library
   to understand the code, possible causes and recommended diagnostic checks.
5. Check live data when necessary.
   Sensor readings, fuel trims and other available live data can help confirm
   the cause of faults involving the air/fuel mixture, sensors, misfires and
   other engine-management systems.
6. Repair the underlying problem and clear the code.
   Clear the fault code only after the cause has been diagnosed and repaired.
   Then drive the vehicle and scan it again if the Check Engine Light returns.

Which diagnostic tool should you use?

The right scanner depends on your vehicle and how much diagnostic access you need.

• iCarsoft single-brand scanners:
  Full-system diagnostics for one supported manufacturer.
• iCarsoft CR PRO:
  Multi-brand full-system diagnostics for users working with several vehicle brands.
  Not sure which iCarsoft model is right for you?
  Compare iCarsoft diagnostic tools and find the right scanner
• OBDeleven PRO:
  Advanced diagnostics and coding for supported Volkswagen Group vehicles.
  Not sure which OBDeleven option you need?
  Compare OBDeleven 3, PRO and ULTIMATE.

Related Guides

• Car misfires
• Car hesitates when accelerating
• Best OBD2 Scanner for Beginners
• OBD2 Scanner Comparison

When to Seek Professional Help

• 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