The check engine light (malfunction indicator light) is part of the on-board diagnostic system (OBD II) that monitors the engine and automatic transmission controls. A steady light means a fault has been detected; Ram’s manual says certain conditions such as a loose or missing gas cap or poor-quality fuel can light it, that the car should be serviced if it stays on through several typical driving styles, and that in most situations the vehicle will drive normally. A flashing light is urgent; see flashing check engine light.
What can make the check engine light come on?
Mazda’s 2025 MX-5 manual lists these triggers:
- A problem with the engine control system
- A problem with the emission control system
- A problem with the transmission control system
- A very low fuel level, or the tank approaching empty
- A fuel-filler cap that is missing or not tightened securely
Ram’s manual also lists a loose or missing gas cap and poor-quality fuel. Neither manual names specific parts; a scan tool reads the stored fault code from the OBD II system. Common codes include P0300 (random misfire) and P0420 (catalyst efficiency). Misfires are often caused by worn spark plugs; see bad spark plug symptoms.
What should you do?
- Check the fuel cap is on and tight, and that you have not run very low on fuel.
- If the light is flashing, treat it as urgent: Ram says severe catalytic converter damage and power loss will soon occur.
- If it stays on, have the car checked. Ram says prolonged driving with the light on could damage the vehicle control system and affect fuel economy and driveability.
- Do not disconnect the battery cables while the light is on or flashing, says Mazda: the engine may be damaged when the cables are reconnected, which could cause a fire.
- If the light does not come on at start-up, have it checked promptly. Ram says the bulb should light when you switch on the ignition before starting.
Frequently asked questions
Can I drive with the check engine light on?
Ram says that in most situations the vehicle will drive normally and will not require towing, but that it should be serviced if the light stays on. If it flashes, Mazda says to park in a safe place immediately.
Will the light go out by itself?
The manuals we read do not say. A tightened fuel cap may clear it after some driving; a stored fault still needs diagnosis.
Sources: Ram, 2022 2500/3500 owner’s manual, “Engine Check/Malfunction Indicator Warning Light (MIL)”; Mazda, 2025 MX-5 owner’s manual, “Check Engine Warning Light” (both read 13 November 2026).
