A CEL, or Check Engine Light, is a signal from your car's onboard computer (the powertrain control module) that it has detected a problem affecting the engine or emissions system. When it illuminates, your first step should be to assess its behavior. A solid light indicates a non-critical issue that should be checked soon, while a flashing CEL signals a severe problem, like a misfire, that could cause immediate damage to the catalytic converter; you should reduce power and seek service immediately.
The light is part of the On-Board Diagnostics II (OBD-II) system, a standardized system in all cars sold in the U.S. since 1996. When a problem occurs, the computer stores a specific Diagnostic Trouble Code (DTC), which a mechanic can retrieve using a scan tool. These codes point to the general area of the fault, but further diagnosis is almost always needed.
Common triggers range from a loose gas cap—one of the most frequent causes—to more serious issues with sensors like the oxygen sensor or mass airflow sensor, or components like the catalytic converter. While you can drive with a solid light, ignoring it can lead to reduced fuel economy, higher emissions, and more costly repairs down the line. The best course of action is to have the codes read, either at an auto parts store (often for free) or by a professional, to understand the urgency.
| Common Trouble Code (DTC) | Potential Cause | Typical Symptom |
|---|
| P0455 | Large Evaporative System Leak (often a loose gas cap) | Solid CEL, possibly faint fuel smell |
| P0420 | Catalyst System Efficiency Below Threshold | Solid CEL, potential loss of power |
| P0300 | Random/Multiple Cylinder Misfire Detected | Flashing or Solid CEL, rough running |
| P0171 | System Too Lean (Bank 1) | Solid CEL, poor acceleration, hesitating |
| P0401 | Exhaust Gas Recirculation (EGR) Flow Insufficient | Solid CEL, engine pinging under load |
