If your check engine light is steady, the car is usually safe to drive for now, but get the code read within a few days. If it’s flashing, the engine is misfiring badly enough to damage the catalytic converter. Slow down, avoid hard acceleration, and stop driving as soon as it’s safe. Check the gas cap first, then get a proper diagnosis.
Quick answer
- Steady light: not an emergency. Drive normally and get it checked soon.
- Flashing light: serious misfire. Pull over when safe and don’t keep driving.
- Red oil pressure or temperature lights with it: stop and shut the engine off.
- A loose gas cap is a common cause, but the light can take a few days of driving to go off after you tighten it.
- A code tells you where to look, not exactly what part to replace.
What does the check engine light mean?
The check engine light, also called the malfunction indicator lamp, comes on when your car’s computer detects a problem with the engine, ignition, fuel, or emissions system. When it turns on, the computer stores a diagnostic trouble code, such as P0301 or P0420, that points to the system involved.
The light itself doesn’t tell you how serious the problem is. It covers everything from a loose gas cap to a failing catalytic converter. What it does tell you is to pay attention, and whether it’s steady or flashing is your first clue.
Steady vs. flashing: is it safe to keep driving?
A steady check engine light means the computer found a fault, but nothing is likely to fail in the next few minutes. You can usually keep driving normally. Still, don’t ignore it for weeks. A small problem like a failing oxygen sensor can hurt fuel economy and, over time, damage more expensive parts.
A flashing check engine light means the engine is misfiring badly. Unburned fuel is going into the exhaust, where it can overheat the catalytic converter and ruin it quickly. If it’s flashing:
- Ease off the gas and slow down. Hard acceleration makes it worse.
- Don’t tow anything or climb long hills if you can avoid it.
- Get off the road when it’s safe and turn the engine off.
- Have the car towed, or drive only a very short distance gently if you have no choice.
Some other signs mean stop right away, no matter what the check engine light is doing:
- The red oil pressure light comes on.
- The temperature gauge goes into the red or a red temperature light comes on.
- You smell burning, see smoke, or hear loud knocking.
- The car suddenly loses power or shakes hard.
What should you check first?
A few simple checks can help before you bring the car in:
- Check the gas cap. Take it off and put it back on until it clicks. Look for a cracked seal. A loose cap causes an evaporative (EVAP) system leak code. The light won’t go off right away. It usually takes a few days of normal driving for the computer to recheck the system.
- Look at the other warning lights. Oil pressure, temperature, battery, and brake lights tell you more about what’s going on and how urgent it is.
- Notice how the car drives. Rough idle, shaking, hesitation, stalling, a loss of power, or worse gas mileage all help narrow things down. Write down when it happens: cold starts, highway speed, at idle, or in the rain.
- Check the oil and coolant. With the engine off and cool, check the oil on the dipstick and the coolant level in the overflow tank. Never open a hot radiator cap.
If everything looks normal and the car drives fine, it’s still worth getting the code read. The light being on also hides new problems, because it can’t come on “again” to warn you about a second fault.
What are the most common causes?
These are the causes we see most often when a check engine light comes on:
- Loose or damaged gas cap. The easiest fix, and a common one.
- EVAP system leaks. Cracked hoses, a bad purge valve, or a vent valve in the fuel vapor system.
- Oxygen sensors. They measure exhaust oxygen to help set the fuel mixture. A bad one hurts fuel economy.
- Spark plugs and ignition coils. Worn plugs or a failing coil cause misfires and are a very common reason for a flashing light. Regular tune-ups help prevent this.
- Mass airflow sensor. A dirty or failing sensor can cause hesitation, rough idle, and poor mileage.
- Catalytic converter. Often a result of other problems, like long-term misfires or burning oil. Our exhaust and muffler repair page covers that part of the system.
- Vacuum leaks and thermostat problems. Cracked vacuum hoses and a thermostat stuck open are also frequent causes.
Is a free code scan enough?
A code reader, including the free scans some parts stores offer, is a good start. It tells you which system flagged the problem. But a code isn’t a diagnosis. A P0420 code, for example, says the catalytic converter isn’t working efficiently, but the real cause might be an oxygen sensor, an exhaust leak, or a misfire. Replacing parts based only on the code can waste money and leave the real problem in place.
A full diagnosis uses a professional scan tool to look at live sensor data, freeze frame data from when the fault happened, and readiness monitors, then tests the suspect parts directly. That’s what our check engine light service and computer diagnostics cover.
One more tip: don’t just clear the code and hope. Clearing it erases useful information, and the light will come back if the problem is still there. It also resets the readiness monitors, which can cause a car to fail an emissions test until it’s been driven enough to set them again. In Maryland, a check engine light that’s on will also keep your car from passing the MVA’s emissions (VEIP) test.
Check engine light still on?
We read the codes, check live data, and test the parts involved so you know the actual cause before you pay for a repair.
Check engine light FAQ
Can I drive with the check engine light on?
If it’s steady and the car drives normally, yes, for a short time. If it’s flashing, or if the oil pressure or temperature light is also on, stop driving as soon as it’s safe.
How long does it take for the light to go off after tightening the gas cap?
Usually a few days of normal driving. The computer has to run its EVAP self-test again before it turns the light off, so it won’t happen right away.
Why is my check engine light flashing?
A flashing light almost always means a severe engine misfire. It can quickly overheat and damage the catalytic converter, so slow down and stop driving when it’s safe.
Will a check engine light go off by itself?
Sometimes. If the problem was temporary, like a loose gas cap, the light can turn off after the computer rechecks the system. The code usually stays stored for a while, so it’s still worth having it read.
Can you check my engine light without an appointment?
Yes. Walk-ins are welcome at our Rockville shop, but calling ahead at (301) 762-1200 gets you in faster.
Bring your car to New Level Auto Service
We’re at 716 E Gude Dr in Rockville, MD. Walk-ins are welcome, and calling ahead gets you in faster.
- Phone (301) 762-1200
- Hours Monday to Friday 8 AM to 5 PM, Saturday 9 AM to 2 PM
- Extras Night drop and 24-hour emergency towing