A car that feels fine can still have a fault
- A steady light means the car's computer has found a fault, and it can do that before you notice anything. Get it checked soon.
- A light that flashes while the engine runs means a severe misfire. Ease off, pull over when it's safe and call roadside assistance.
- If your car has a gas cap, make sure it's on and tight, with the car parked and the engine off. A loose cap can turn the light on.
- Get the codes read, and don't clear them before the shop sees them.
- Oil pressure or overheating warning as well? Stop as soon as it's safe and switch the engine off.
It's tempting to ignore a check engine light when the car drives perfectly well, but the computer can pick up a problem before you'd notice anything.
We'd work out whether the light is steady or flashing first, because that decides whether you book a check or pull over. After that, check the gas cap and get the codes read.
If it's flashing or other warnings come on
A check engine light that flashes while the engine is running points to a severe misfire, and even a short stretch of that can seriously damage the catalytic converter. Ford's Super Duty warning lamp instructions (opens in a new tab) tell owners to avoid heavy acceleration and deceleration and have the vehicle checked immediately.
We'd go a step further and stop driving. Ease off the gas, pull over when it's safe, and call roadside assistance.
The flashing that matters is the kind you see with the engine running. A brief flash when you first switch the ignition on is the system getting ready to check the car, and on a 2020 Ford Super Duty, if the light flashes eight times after 15 seconds with the ignition on and the engine not yet started, the truck isn't ready for an emissions inspection.
Other warnings change the plan too, however smooth the engine feels. For an oil pressure or engine overheating warning, stop as soon as it's safe and switch the engine off. Chevrolet adds that an overheated car should be towed in for service.
Start with the gas cap
If your car has a gas cap, check that it's there and tightened, with the car parked and the engine off. A loose or missing cap lets fuel vapor escape, and the computer reads that as a leak.
Some cars have their own loose-cap warning as well, like the “Fuel filler cap not properly closed” light on Volkswagens or the Check Fuel Cap message on a 2024 RAV4.
Tightening the cap won't switch the light off right away. EPA says it should go out within a few days, and its check engine light brochure (opens in a new tab) says to have the car serviced if the light is still on after a few short trips.
Get the codes read and keep them
The light comes with a stored trouble code that helps a technician find the problem. A repair shop or dealer reads it with an OBD scan tool.
If you use your own code reader, do it with the car in Park (Neutral for a manual) and the parking brake set, in a well-ventilated spot. Block the drive wheels, stay with the car, and keep clear of moving or hot engine parts.
We'd write the codes down and leave them in the car's memory for the shop. Erasing them also wipes the readiness status that emissions inspections check, and on some cars a disconnected battery does the same.
In California, for example, a car won't pass a Smog Check (opens in a new tab) with the check engine light on, and clearing codes or disconnecting the battery doesn't remove the permanent codes the test also looks for on 2010 and newer cars, apart from some 2010 models that don't support those codes. Mandatory state emissions programs have to include an OBD check, so look up your own state's rules before your test.
What can cause it when the car feels normal
The gas cap is the one cause you can check yourself. Everything else needs a scan and a technician's diagnosis.
Some faults leave the car feeling completely normal. Ford says a failed oxygen sensor or evaporative (charcoal) canister can show no noticeable symptoms beyond the light, even though the car runs less efficiently and puts out more emissions.
A poorly performing spark plug can cause a misfire you don't notice, and that misfire can quickly degrade the catalytic converter.
If you drive a 2024 Toyota RAV4
On a 2024 U.S. gasoline RAV4, the check engine light is the malfunction indicator lamp. It signals a problem in the electronic engine control, electronic throttle control or emission control system (if equipped), and Toyota's warning light instructions (opens in a new tab) are to have the car inspected by a Toyota dealer immediately. A normal-feeling drive doesn't change that. If yours is a hybrid, use the hybrid's own manual.
Toyota says to stop immediately in a safe place for these warnings, whatever the check engine light is doing:
- Low engine oil pressure, meaning the pressure is too low. Also contact your Toyota dealer.
- High coolant temperature, meaning the engine is overheating. Toyota's overheating steps have you turn off the air conditioning, then stop the engine.
- The red brake system warning, for low brake fluid or a brake system malfunction. Also contact your Toyota dealer, because continuing to drive may be dangerous.
- The charging system warning, for a charging system malfunction. Also contact your Toyota dealer.
The oil pressure and coolant warnings also show a message on the multi-information display.
Toyota's first checks when the light comes on while you're driving are an empty tank and a loose fuel cap. Fill an empty tank right away, and if you've run out of fuel, call roadside assistance. To check the cap:
- Turn the engine switch to OFF and close the doors and windows.
- Touch an unpainted metal surface to discharge static electricity before you open the fuel door, and don't smoke.
- Pull up the opener to open the fuel door. If the cap is loose, leave it in place and turn it until you hear a click. When you let go, it turns slightly back the other way. Close the fuel door when you're done.
A Check Fuel Cap message gets the same fix: engine switch to OFF, then check the cap and tighten it securely. Topping off a nearly full tank again and again can also turn the light on by mistake, so stop filling when the nozzle clicks off.
After you fill an empty tank or tighten a loose cap, the light goes off after several driving trips. If it's still on after several trips, contact your Toyota dealer as soon as possible. Don't take extra drives just to clear it, and keep the dealer inspection on your list either way.
What to tell the shop
When you call, ask how they want the car brought in, including whether it should be towed. If the light starts flashing or new warnings or symptoms show up, stop and call the shop or roadside assistance before you carry on.
Have these ready:
- The year, make, model and engine
- Whether the light is steady or flashing, plus the exact wording of any dashboard message
- Any other warnings, rough running or loss of power you've actually noticed
- Recent fueling, whether the tank ran empty, and whether you found and tightened a loose cap
- The codes, if they've been read, and whether anyone cleared them
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