If your vehicle is showing check engine light stays on solid, here are the most likely causes and the parts you'll need to fix each one.
The most common and simplest reason for a check engine light is a loose, damaged, or missing gas cap. This creates a small leak in the evaporative emissions (EVAP) system.
Oxygen sensors degrade over time and fail to accurately measure exhaust gases. This reduces fuel economy and triggers the emissions monitors.
The MAF sensor measures air entering the engine to calculate fuel delivery. It can get dirty, fail, or commonly be left unplugged after checking the engine air filter.
Worn ignition components cause engine misfires, which trigger the light and can cause rough idling. This can also happen if a coil or wire isn't seated properly after a tune-up.
If the gas cap is fine, the evaporative emissions system may have a failing purge or vent valve, preventing fuel vapors from being properly routed to the engine.
The catalytic converter neutralizes harmful exhaust gases. If it fails, usually due to age or unburned fuel from misfires, it will trigger a persistent check engine light.
Low or extremely dirty engine oil can clog the VVT solenoids, preventing the engine from adjusting valve timing properly. Using the wrong oil viscosity during an oil change can also cause this.