Why is my check engine light on?

A check engine light means your car's computer logged a fault code — anywhere from a loose gas cap (cheapest fix on the planet) to a failing catalytic converter ($1,500+). The only way to know which is which is to read the OBD2 code. Any auto-parts store will scan it for free, or plug in a $20 Bluetooth scanner. The code itself (e.g. P0420, P0171) tells you the system involved before you spend a dime.

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