The check engine light means your car's onboard diagnostic system detected a problem with the engine, emissions system, or transmission

When that light comes on—usually a small engine icon on your dashboard—it does not mean your engine is about to fail or that you are in when ready danger. It means your car's computer found a code that suggests something is not working as designed. The problem could be minor, like a loose gas cap, or more serious, like a faulty oxygen sensor. The light itself cannot tell you which one.

The light will stay on until the problem is fixed and the code is cleared from the car's memory. Some cars let you clear it yourself; others require a mechanic or dealer to do it. If you clear the code without fixing the underlying problem, the light will come back on.

Key Takeaways

  • A check engine light means your car's diagnostic system found a fault code, but the light alone does not tell you what the problem is.
  • You need a code reader to find out what triggered the light—many auto parts stores will read the code for free.
  • A flashing check engine light means stop driving when ready; a steady light means you can usually drive to a mechanic, but do not ignore it.
  • Common causes include a loose gas cap, a bad oxygen sensor, a faulty catalytic converter, or a misfire in one of the cylinders.
  • The light will not go away on its own once the code is set, even if the problem fixes itself.

How to read the code yourself

You cannot see what the code is just by looking at the light. You need a tool called an OBD-II scanner (On-Board Diagnostics, version 2). This device plugs into a port under your steering wheel or dashboard—usually on the left side below the wheel—and reads the fault code stored in your car's computer.

Many auto parts stores, including AutoZone, O'Reilly Auto Parts, and Advance Auto Parts, will plug in a scanner and read your code for free. You can also buy a basic scanner yourself for $25 to $100. Some newer cars let you check the code through the car's own touchscreen menu, though not all do. Once you have the code—it will be something like P0420 or P0171—you can search online to see what it means and how serious it typically is.

Flashing versus steady light

A flashing check engine light is more urgent than a steady one. A flashing light means your engine is misfiring—one or more cylinders are not firing correctly—and unburned fuel is going into your exhaust system. This can damage your catalytic converter, which is expensive to replace. If your light is flashing, stop driving as soon as safely possible and have the car towed or driven directly to a mechanic.

A steady check engine light usually means the problem is not an emergency, but you should still have it checked within a few days. Driving with a steady light for weeks or months can sometimes make the underlying problem worse, and it may affect your fuel economy or engine performance.

Common reasons the light comes on

The most common cause is a loose or missing gas cap. Your fuel system is sealed, and if the cap is not tight, the computer detects a leak. Tighten the cap, drive for a few minutes, and the light may go off on its own. If it does not, the code reader will tell you whether it was the cap or something else.

A faulty oxygen sensor is the second most common trigger. This sensor measures how much oxygen is in your exhaust and helps the engine run efficiently. When it fails, the light comes on, but the car is usually still drivable. Replacing an oxygen sensor typically costs $150 to $300.

A bad catalytic converter is more expensive to fix—usually $400 to $1,200 or more—but it does not always cause when ready symptoms. You might notice reduced fuel economy or a rotten-egg smell from the exhaust. A loose or damaged spark plug wire or a misfire in one cylinder can also trigger the light and is usually cheaper to repair.

What to do when the light comes on

First, check that your gas cap is tight. If the light stays on, get the code read at an auto parts store or with your own scanner. Write down the code number so you have it when you call a mechanic.

If the light is steady and your car is running normally, you can usually drive it to a mechanic within a few days. If the light is flashing, the car is making strange noises, or you feel a loss of power, do not drive it—have it towed instead. Continuing to drive with a serious problem can cause additional damage.

When you take the car to a mechanic, tell them the code number and when the light came on. A good mechanic will not just clear the code; they will diagnose what caused it and fix the actual problem. If you go to a dealer, the cost will usually be higher than an independent mechanic, but the diagnosis is often more thorough.

Can you ignore the light?

You can drive with the light on for a while, but ignoring it is risky. The problem could get worse and more expensive to fix. In some states, a car with a check engine light will fail an emissions test and you cannot renew your registration. If you plan to sell the car, a potential buyer will likely have it scanned, and a fault code will lower the price or kill the deal.

The light is your car's way of telling you something needs attention. It might be small, but it might not be. Getting it checked costs little compared to the cost of ignoring a real problem.

Clearing the code yourself versus having a mechanic do it

Once the underlying problem is fixed, the code needs to be cleared from the car's computer so the light goes off. Some mechanics will clear it as part of the repair. If you have a code reader, you can clear it yourself—most scanners have a "clear codes" button. However, if you clear the code without fixing the problem, the light will come back on within a few driving cycles.

A few cars will clear the code automatically after you fix the problem and drive for a certain number of miles, but most will not. If the light stays on after you have fixed what you think is wrong, the code reader will tell you whether the original problem is still there or whether a new one has appeared.

Frequently Asked Questions

Is it safe to drive with the check engine light on?

If the light is steady and your car is running normally, yes—you can usually drive it to a mechanic within a few days. If the light is flashing, your car is making unusual noises, or you feel a loss of power, do not drive it. A flashing light means engine damage is happening right now.

Will the check engine light go away on its own?

No. Once a fault code is set, the light stays on until the code is cleared. Even if the problem fixes itself, the light will not turn off. You have to clear the code with a scanner or have a mechanic do it.

How much does it cost to fix a check engine light?

It depends entirely on what caused it. A loose gas cap costs nothing to fix. An oxygen sensor might cost $150 to $300. A catalytic converter can cost $400 to $1,200 or more. The only way to know is to have the code read and diagnosed.

Can I clear the code and ignore the problem?

You can clear it, but the light will come back on within a few days or weeks if the underlying problem is not fixed. You will end up clearing it repeatedly, and the problem may get worse and more expensive to repair in the meantime.

What if the code reader shows multiple codes?

Multiple codes usually mean one problem triggered several sensors. For example, a misfire in one cylinder might set a misfire code, an oxygen sensor code, and a catalytic converter code all at once. A mechanic can usually figure out which one is the root cause and fix it, which clears the others.