A steady check engine light means the OBD2 system has stored a fault code for a problem that is not immediately severe. You can typically drive to a shop within a few days. A flashing check engine light is an emergency: it indicates an active engine misfire that is sending unburned fuel into the hot catalytic converter. The catalytic converter can be destroyed in minutes of driving with a flashing CEL — turning a $150 spark plug repair into a $1,500 catalytic converter replacement. If the light is flashing, pull over and have the vehicle towed or drive it directly to a shop at low speed without delay.
The check engine light tells you something is wrong — the OBD2 fault code tells you what. To read the code, plug an OBD2 scanner into the diagnostic port under the driver's side dash. Free code reads are available at AutoZone, O'Reilly Auto Parts, and Advance Auto Parts. Basic Bluetooth OBD2 scanners for smartphones cost $20 to $50 and are worth owning for any driver. The code is a 5-character identifier (P0420, P0171, P0300) that points to the affected system. From there, look up the specific code for causes, urgency, and repair procedure.
P0420 — Catalytic converter efficiency below threshold. Often triggers after 100,000 miles. Can indicate a failing converter, oxygen sensor fault, or exhaust leak upstream. P0171 — System too lean Bank 1. Causes: vacuum leak, dirty MAF sensor, failing fuel pressure regulator. P0300 — Random multiple cylinder misfire. Usually spark plugs, ignition coils, or vacuum leak. P0442 — Small EVAP leak detected. Often just a loose or worn gas cap. P0128 — Coolant temperature below thermostat regulating range. Usually a stuck-open thermostat.
A flashing check engine light means an active engine misfire is occurring right now. Unburned fuel is entering the catalytic converter and can destroy it within minutes of driving. This turns a $150 spark plug repair into a $1,500+ catalytic converter replacement. If the light is flashing, pull over and have the vehicle towed or drive directly to a shop at low speed.
A steady (non-flashing) check engine light means a non-critical fault code is stored. You can typically drive 50 to 200 miles to reach a shop for diagnosis. A flashing check engine light means stop driving — active engine damage is occurring. Scan the code for free at any auto parts store to identify the problem.
Plug an OBD2 scanner into the diagnostic port under the driver side dashboard. Free code reads are available at AutoZone, O'Reilly Auto Parts, and Advance Auto Parts. Basic Bluetooth OBD2 scanners for smartphones cost $20 to $50. The code is a 5-character identifier (like P0420 or P0171) that points to the affected system.
P0420 (catalytic converter efficiency), P0171 (system too lean Bank 1), P0300 (random misfire), P0442 (small EVAP leak, often a loose gas cap), and P0128 (coolant temperature below thermostat range). These five codes account for the majority of check engine light triggers across all vehicles.
Reviewed by Vukašin Herbez — Automotive Editor · Last updated 2026-11-13