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Check engine light basics

General info, not professional advice. These are common causes that apply to most cars, not a diagnosis of yours. If you are unsure or a problem affects brakes, steering, airbags or fuel, see a qualified mechanic.

Likely causes (most common first)

1. Loose or bad gas cap
   A common cause of evaporative system codes.
   🔎 Find a used gas cap
2. Oxygen sensor
   Often comes with worse fuel economy.
   🔎 Find a used oxygen sensor
3. Misfire (plugs, coils)
   Rough running. A FLASHING light means a severe misfire.
   🔎 Find a used ignition coil
4. Mass air flow sensor
   Hesitation or a rough idle.
   🔎 Find a used mass air flow sensor
5. Catalytic converter
   Often shows up after other problems are ignored, and can come with a rotten-egg smell.
   🔎 Find a used catalytic converter

Quick safe checks

• Tighten the gas cap until it clicks.
• Read the code with an OBD-II scanner (1996 and newer US cars). Many parts stores read codes for free.
• Look up what the code means, but treat it as a starting point. A code names a system, not always the failed part.

When to see a pro

The light is FLASHING (pull over and reduce load: severe misfires can damage the catalytic converter), the car runs very rough, or temperature or oil pressure warnings come on too.

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