Check Engine Light On: What to Do First
A steady check engine light is rarely an emergency; a flashing one is. What to check first, how to read the code yourself, and when to stop driving.
On this page· 8 sections
A check engine light is your car's emissions and engine computer telling you it has stored a fault. It might be a loose gas cap, a sensor that's drifting, or a misfire that's hurting the catalytic converter. The light alone can't tell you which. What it does tell you, by whether it's steady or flashing, is how urgently you need to find out.
First: steady or flashing?
This is the most important thing to notice, so look before you do anything else.
| What you see | What it usually means | What to do |
|---|---|---|
| Steady amber light | A fault has been stored. Often not urgent. | Check the gas cap, read the code, book a diagnosis soon. Drive normally if the car feels normal. |
| Flashing amber light | An active, severe misfire. Unburned fuel can overheat and damage the catalytic converter. | Slow down, avoid hard acceleration and stop driving as soon as it's safe. Get it diagnosed before driving further if you can. |
| Light plus a red warning (oil pressure, temperature, brakes) | The red light is the priority. | Pull over safely and switch off. Don't keep driving on a red warning. |
- Steady amber light
- What it usually means
- A fault has been stored. Often not urgent.
- What to do
- Check the gas cap, read the code, book a diagnosis soon. Drive normally if the car feels normal.
- Flashing amber light
- What it usually means
- An active, severe misfire. Unburned fuel can overheat and damage the catalytic converter.
- What to do
- Slow down, avoid hard acceleration and stop driving as soon as it's safe. Get it diagnosed before driving further if you can.
- Light plus a red warning (oil pressure, temperature, brakes)
- What it usually means
- The red light is the priority.
- What to do
- Pull over safely and switch off. Don't keep driving on a red warning.
Also notice how the car is behaving: rough idle, hesitation, stalling, a new noise, or a drop in fuel economy. Write it down. Symptoms plus the code are what a mechanic needs.
Check the gas cap
On cars with an evaporative emissions (EVAP) system, which is nearly all modern gas cars, a cap that isn't tightened until it clicks can let fuel vapor escape and set a code. It's the cheapest fix there is, so start here.
- Remove the cap and check the seal for cracks or debris.
- Refit it and turn until it clicks (or as your manual describes; many capless fillers have no cap to check).
- Drive normally for a few days. The light won't go out immediately: the car has to re-run its leak test over several drive cycles.
If the light stays on after a week or so, or comes back, read the code.
Read the code yourself
Every car and light truck sold in the US since the 1996 model year has a standard OBD-II diagnostic port. It's a 16-pin socket under the dashboard on the driver's side, usually near the steering column or above the pedals.
- A basic code reader plugs into the port, reads the stored code and its description, and can show freeze-frame data (engine conditions when the fault was stored). Bluetooth adapters do the same through a phone app.
- Many auto parts stores will read the code for free in the parking lot.
- A shop will scan it as part of a diagnosis, usually for a fee that covers more than reading the code.
Codes look like P0420: a letter and four digits.
| Part | Meaning |
|---|---|
| First letter | P powertrain (engine, transmission), B body, C chassis, U network/communication. The check engine light is mostly P codes. |
| First digit | 0 generic code, same meaning on every car. 1 manufacturer-specific. (2 and 3 are split between the two.) |
| Second digit | The subsystem. For generic P0 codes: 1-2 fuel and air metering, 3 ignition and misfires, 4 auxiliary emissions (EGR, EVAP, catalyst), 5 idle and speed control, 6 computer and outputs, 7-8 transmission. |
| Last two digits | The specific fault. |
- First letter
- Meaning
- P powertrain (engine, transmission), B body, C chassis, U network/communication. The check engine light is mostly P codes.
- First digit
- Meaning
- 0 generic code, same meaning on every car. 1 manufacturer-specific. (2 and 3 are split between the two.)
- Second digit
- Meaning
- The subsystem. For generic P0 codes: 1-2 fuel and air metering, 3 ignition and misfires, 4 auxiliary emissions (EGR, EVAP, catalyst), 5 idle and speed control, 6 computer and outputs, 7-8 transmission.
- Last two digits
- Meaning
- The specific fault.
Enter a VIN, a model like “2018 Accord”, or a code like P0420. Free, no signup.
Common codes and what they point to
These are among the codes owners look up most. Each links to its full page with symptoms, causes and diagnostic steps.
| Code | Description | Often caused by |
|---|---|---|
| P0420 | Catalyst system efficiency below threshold (bank 1) | An aging catalytic converter, but also oxygen sensor faults or exhaust leaks. Check those before replacing the converter. |
| P0300 | Random or multiple cylinder misfire | Worn spark plugs, ignition coils, vacuum leaks or fuel delivery. A flashing light often comes with this one. |
| P0171 | System too lean (bank 1) | Vacuum leaks, a dirty mass airflow sensor, or low fuel pressure. |
| P0455 | EVAP system large leak detected | A loose or damaged gas cap, or a cracked EVAP hose. |
| P0442 | EVAP system small leak detected | A worn cap seal or a small hose or valve leak. |
| P0128 | Coolant temperature below thermostat regulating temperature | A thermostat stuck open, or a coolant temperature sensor fault. |
- Description
- Catalyst system efficiency below threshold (bank 1)
- Often caused by
- An aging catalytic converter, but also oxygen sensor faults or exhaust leaks. Check those before replacing the converter.
- Description
- Random or multiple cylinder misfire
- Often caused by
- Worn spark plugs, ignition coils, vacuum leaks or fuel delivery. A flashing light often comes with this one.
- Description
- System too lean (bank 1)
- Often caused by
- Vacuum leaks, a dirty mass airflow sensor, or low fuel pressure.
- Description
- EVAP system large leak detected
- Often caused by
- A loose or damaged gas cap, or a cracked EVAP hose.
- Description
- EVAP system small leak detected
- Often caused by
- A worn cap seal or a small hose or valve leak.
- Description
- Coolant temperature below thermostat regulating temperature
- Often caused by
- A thermostat stuck open, or a coolant temperature sensor fault.
Decide what to do next
Once you know the code:
- Look it up on our OBD-II code pages for causes in order of likelihood, symptoms and what a fix typically involves.
- Check for recalls and service bulletins. Some faults are covered by a recall or a known fix. Start with the recalls for your make and model, then confirm with a dealer.
- Check your warranty. Beyond the factory warranty, federal law requires an emissions warranty: generally 2 years or 24,000 miles on emission-related parts, and 8 years or 80,000 miles on major components such as the catalytic converter and the engine computer. California and some other states require longer coverage. Your warranty booklet lists exactly which parts are covered and for how long.
- Fix it or book it in. Simple jobs (gas cap, spark plugs on many engines, a mass airflow sensor clean) are within reach of a careful DIYer. For anything else, get a diagnosis and a written quote; you can send the job with the code to a shop through findmechanic.ai (opens in a new tab).
Don't just clear the code
A reader can turn the light off, and it's tempting. Resist it unless you've fixed the problem:
- You erase the evidence. Clearing removes the stored code and freeze-frame data that help diagnose an intermittent fault.
- The light will come back if the fault is still there, sometimes after days.
- You can fail an emissions inspection. Clearing codes resets the car's readiness monitors. Many inspection programs won't pass a car until enough monitors have completed, which takes a mix of city and highway driving.
Disconnecting the battery has the same effect and can also reset radio codes and learned idle settings.
Keep a record of the code
Write down the code, the date, the mileage and what was done about it. If the light returns months later, the history makes diagnosis faster and shows a repeat fault under warranty. A free CarManager garage lets you log codes and repairs against your car alongside its service history.
Frequently asked questions
Can I drive with the check engine light on?
If the light is steady and the car drives normally, it's usually fine to drive carefully to a diagnosis in the next few days. If it's flashing, or the car is running badly, slow down and stop driving as soon as it's safe: a severe misfire can damage the catalytic converter quickly.
Can a loose gas cap really turn on the check engine light?
Yes. On cars with an EVAP leak test, a loose or worn cap can set a leak code such as P0455 or P0457. Tighten it until it clicks and drive for a few days; the light should go out on its own once the test passes.
How do I find out why my check engine light is on?
Read the stored trouble code with an OBD-II reader (many parts stores do this free), then look the code up. The code tells you which system flagged a fault; a diagnosis confirms the cause.
Will the check engine light reset itself?
Often, yes. If the fault goes away, the light usually turns off after the car's self-test passes over several drive cycles. The code stays in memory as a history code for a while.
Is the check engine light covered by warranty?
The light isn't, but the repair may be. Faults in emission-related parts can be covered by the federal emissions warranty, which can outlast the basic factory warranty: generally 2 years or 24,000 miles for emission parts and 8 years or 80,000 miles for major components like the catalytic converter.
Key takeaways
- Steady light means "get it checked soon". Flashing light means a severe misfire: slow down and stop driving as soon as it's safe.
- Check the gas cap first, then read the code. The code is the starting point of a diagnosis, not the answer.
- Many parts stores read codes for free, and a basic OBD-II reader costs less than most diagnostic fees.
- Don't just clear the code. You erase useful data and can fail an emissions inspection.
- Emissions parts carry a federal warranty that can outlast the factory warranty.
Look it up on CarManager
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General guidance, not professional advice. Intervals, specifications and legal requirements vary by vehicle, manufacturer and state: your owner’s manual, a qualified mechanic and your state agency come first. Costs mentioned are estimates. Our data sources.