Quick Answer
A steady check engine light means the car has stored a fault that should be diagnosed soon, and you can usually keep driving carefully in the meantime. A flashing light usually means an active engine misfire that can quickly damage the catalytic converter. Ease off the throttle, avoid hard driving, and get to a shop right away.
Questions about your car? Call us or book online. Walk-ins are welcome.
Call (415) 440-5353| What you see | What it usually means | What to do |
|---|---|---|
| Steady light, car drives normally | The computer has stored a fault code. Often something minor, sometimes not. | Check the gas cap, then book a diagnosis within the next few days. |
| Steady light, car runs rough, hesitates, or gets worse mileage | A real problem is developing, such as a misfire or a failing sensor. | Get it diagnosed this week, and sooner if it's getting worse. |
| Flashing light | An active misfire. Unburned fuel can overheat and damage the catalytic converter. | Reduce speed and load, and get to a shop right away. Don't keep driving long distances. |
- What it usually means
- The computer has stored a fault code. Often something minor, sometimes not.
- What to do
- Check the gas cap, then book a diagnosis within the next few days.
- What it usually means
- A real problem is developing, such as a misfire or a failing sensor.
- What to do
- Get it diagnosed this week, and sooner if it's getting worse.
- What it usually means
- An active misfire. Unburned fuel can overheat and damage the catalytic converter.
- What to do
- Reduce speed and load, and get to a shop right away. Don't keep driving long distances.
If your check engine light is flashing
A flashing light is the car's way of saying something is wrong right now. In most cases that means one or more cylinders are misfiring. Unburned fuel gets pushed into the exhaust, where it can overheat the catalytic converter, one of the most expensive parts in the exhaust system.
- 1Take your foot off the gas gently and slow down. Avoid hard acceleration, towing, and steep hills while the light is flashing.
- 2Find a safe place to pull over if the engine is shaking, losing power, or making unusual noises.
- 3Call a shop and tell them the light is flashing. If the car runs very rough or you smell something like rotten eggs, don't keep driving. Arrange a tow instead.
- 4Don't just clear the code and carry on. The misfire is still there.
If your check engine light is steady
A steady light is less urgent, but don't ignore it. It means the car's computer has detected a problem and stored a trouble code. Here's a quick checklist before you call a shop:
- Check the gas cap. A loose or worn cap can trigger the light. Tighten it until it clicks. If that was the cause, the light can take a day or two of normal driving to turn off.
- Notice how the car feels. Rough idle, hesitation, stalling, a drop in fuel economy, or trouble starting all point to something that needs attention soon.
- Note when it started. Did the light come on right after filling up, driving through heavy rain, or hitting a big pothole? That's useful information for the technician.
- Keep driving gently until you can get in. If the car feels normal, a short trip to the shop is usually fine. If it feels different, don't wait.
Common causes of a check engine light
There are hundreds of possible trouble codes, but a handful of causes show up again and again:
- A loose, damaged, or worn gas cap (the cheapest fix there is)
- Failing spark plugs or ignition coils, which cause misfires
- A worn oxygen sensor or mass airflow sensor
- A leak in the fuel vapor (EVAP) system
- A failing catalytic converter, often the result of an ignored misfire. See our exhaust system service.
| Item | Typical range |
|---|---|
| Check engine light diagnosis | About $100 to $200 |
| Ignition coil replacement | About $150 to $450 per coil, parts and labor |
| Catalytic converter replacement | Often $1,000 to $2,500 or more, depending on the vehicle |
- Typical range
- About $100 to $200
- Typical range
- About $150 to $450 per coil, parts and labor
- Typical range
- Often $1,000 to $2,500 or more, depending on the vehicle
These are general market ranges from published repair-cost guides. Exact prices depend on your make and model and on what's actually wrong. That's why a proper diagnosis comes first, and why catching a misfire early usually costs far less than replacing a converter later.
Why a free code read isn't a diagnosis
Many parts stores will read your trouble code for free, and it's a useful starting point. But a code tells you which system saw a problem, not which part has failed. A misfire code, for example, could be a spark plug, a coil, a fuel injector, or something else entirely. Replacing parts based on the code alone is how people end up spending money without fixing the problem.
A proper diagnosis goes further. Our technicians use factory-level scan tools to read the codes, look at live data and freeze-frame information, and run tests to find the specific component that failed. See our diagnostics and electrical service for what's included.
Why you shouldn't just clear the code
Disconnecting the battery or using a scan tool to clear the code turns the light off, but it doesn't fix whatever caused it. If the problem is still there, the light will come back, and you'll have lost the stored information a technician could have used. A misfire that's ignored can also turn a modest repair into a very expensive one.
What to do in San Francisco
Hills and stop-and-go traffic ask a lot of an engine, and a misfire you barely notice on a flat street can get much worse on a climb or when merging onto US-101 or I-280. If your light is flashing, stay off the hills and the freeway and drive gently on flat streets to the nearest shop. If it's a steady light, we're happy to take a look. Walk-ins are welcome, or you can book online.
Frequently Asked Questions
Is it safe to drive with a flashing check engine light?
It's best not to. A flashing light usually means an active misfire, which can damage the catalytic converter if you keep driving. If you must move the car, drive gently and as little as possible, and get it to a shop right away. If it's shaking badly, losing power, or you smell rotten eggs, pull over and arrange a tow.
How long can I drive with a steady check engine light?
If the car drives normally, you can usually keep driving for a few days, but get it diagnosed soon. Small problems that go unfixed can get bigger and more expensive. If the car starts running rough or the light begins to flash, stop driving it and call a shop.
Can a loose gas cap really turn on the check engine light?
Yes. A loose, damaged, or missing gas cap can let fuel vapor escape and trigger the light. Tighten it until it clicks. If that was the cause, the light may take a day or two of normal driving to go off. If it stays on, there's likely something else going on.
Why did my check engine light come on and then go off by itself?
Some faults come and go, and the computer will turn the light off if it doesn't see the problem again for several drive cycles. The code usually stays stored, though, so it's still worth having a technician take a look, especially if you noticed any change in how the car drove.
How much does a check engine light diagnosis cost?
Typical market prices run around $100 to $200, and some shops charge more for complex problems or certain vehicles. The exact price depends on your make and model and on the nature of the issue. Call us or stop by and we'll explain what to expect.
Related Services
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