What Does the Check Engine Light Mean?

Driving & Independence · Explainer · 4 min read

What Does the Check Engine Light Mean?

The little amber engine symbol on the dashboard causes more worry than almost any other warning light, mostly because it says so little. It does not mean the car is about to fail. It means the car's computer has noticed something outside its normal range and stored a code explaining what. The job is to find out which, and how soon it matters.

Solid or flashing: the one thing to check first

A solid light means a fault has been recorded that is not an emergency. The car is usually safe to drive normally for a short time, but it should be looked at within the next few days.

A flashing light is different. It usually means the engine is misfiring badly enough that unburned fuel is reaching the exhaust, which can overheat and damage the catalytic converter within minutes. Ease off the accelerator, avoid towing or climbing hills, and get off the road somewhere safe. If the flashing continues, or the car shakes, loses power or smells hot, stop and call for roadside help rather than driving on.

If the check engine light comes on together with a red warning, such as oil pressure or temperature, treat the red light as the priority and stop as soon as it is safe.

The most common causes

• A loose or worn gas cap. The simplest and one of the most frequent. The fuel system is sealed, and a cap that is not tightened until it clicks lets vapor escape.
• Oxygen sensor. It measures the exhaust so the engine can adjust its fuel mix. A failing one quietly costs fuel economy.
• Spark plugs or ignition coils. The usual source of a misfire, and the usual reason for a flashing light.
• Mass airflow sensor. Measures air coming into the engine; dirt or a fault causes rough running or stalling.
• Catalytic converter. Often the end result of an earlier problem left too long, and one of the more expensive repairs.
• Evaporative (EVAP) system leaks. Small cracked hoses or valves in the fuel vapor system, closely related to the gas cap fault.

What to do, in order

1. Note what the light is doing and anything else you noticed: shaking, a smell, a noise, lower power. Write it down; it helps the mechanic.
2. Check the gas cap. Take it off, check the rubber seal for cracks, and refit it until it clicks. The light may take several days of normal driving to go out on its own.
3. Get the code read. Every car and light truck sold in the US since 1996 has a standard diagnostic port under the dashboard. Many auto parts stores will read the code free of charge, or a mechanic can do it as part of an inspection.
4. Book the repair based on what the code says. A solid light with a known minor cause can wait for a convenient appointment; a misfire should not.

Avoid simply clearing the code and carrying on. The light will return if the fault is still there, and a state emissions or safety inspection will usually fail a car with the light on.

For families of older drivers

A dashboard light is a common reason for an older parent to stop driving to appointments, or to keep driving and not mention it. Neither is ideal. A few things make it easier:

• Keep a small card in the glove box with the number of a trusted mechanic and the roadside assistance line.
• Agree on a simple rule: flashing means pull over and call; solid means call within a day or two.
• Offer to take the car in, or to be on the phone during the visit. Many people find mechanic conversations stressful, and having a second person listen helps.
• Put a phone charger and a flashlight in the car, so a stop at night is not made worse by a dead phone or a dark roadside.

Worth keeping in the car

A breakdown is rarely dangerous in itself; waiting beside the road in the dark is the uncomfortable part. Three small things help:

• A flashlight that is always charged. The ReadyLight 3-in-1 Emergency Flashlight lives in a wall socket at home, so it is charged when you grab it for the car.
• A way to draw attention. The CallOut Personal Safety Alarm has a 130dB siren and an LED light, useful for a roadside stop or a dark parking lot.
• A comfortable seat for longer drives. The SitEase Memory Foam Seat Cushion takes pressure off the lower back and tailbone on the drive to the garage.

For what to do when the power goes out at home, see Power Cuts, Alarms and Getting Help Fast.

The short version

Solid: not urgent, get the code read this week. Flashing: slow down, stop somewhere safe and get help. Start with the gas cap, keep notes, and never ignore a light that keeps coming back.

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