A check engine light means the car's engine computer has detected a fault in the engine, fuel, ignition or emissions systems and stored a diagnostic trouble code. A steady light means the problem should be checked soon. A flashing light usually means a severe misfire that can overheat and damage the catalytic converter, so reduce speed and have it checked right away. Reading the code is only the first step: the code tells you which system or sensor reported a problem, not which part to replace. In metro Atlanta's emissions counties, an illuminated check engine light will also cause a vehicle to fail its emissions inspection.
What the light is connected to
Cars and light trucks sold in the United States since the 1996 model year use a standardized on-board diagnostics system known as OBD-II. The engine control module watches dozens of sensors, including oxygen sensors, airflow, engine temperature, misfire detection and the evaporative emissions system. When a reading falls outside its expected range under certain conditions, the module stores a code and may turn on the malfunction indicator lamp, which most people call the check engine light. A standard connector under the dashboard, usually near the steering column, lets a scan tool read the stored codes and live data.
Steady versus flashing
| Light | What it usually means | What to do |
|---|---|---|
| Steady | A fault has been detected; may affect emissions, fuel economy or performance | Check the gas cap, then have codes read within days |
| Flashing | Active misfire severe enough to damage the catalytic converter | Reduce speed and load; avoid hard acceleration; get it checked promptly; stop if the car runs very roughly |
| Light plus other warnings (oil, temperature, battery) | Possibly a more serious problem | Pull over safely and check; the other warning may be more urgent |
Common causes
- Loose, damaged or missing gas cap. It can trigger an evaporative system leak code. Tighten it until it clicks; the light may take several drive cycles to go out.
- Oxygen sensor faults. These sensors measure exhaust oxygen to fine-tune fuel mixture and wear over time.
- Ignition misfires. Worn spark plugs, failing ignition coils or plug wires.
- Mass airflow sensor problems. Often from contamination or a dirty air filter.
- Catalytic converter efficiency codes. Sometimes a failing converter, sometimes an upstream problem such as misfires or a sensor fault.
- Vacuum leaks. Cracked hoses or intake gaskets letting in unmeasured air.
- Thermostat stuck open. The engine runs too cool, which can set a code.
- EGR or evaporative system valves. Common on many vehicles as they age.
Code reading versus diagnosis
Many auto parts stores will read codes for free, and inexpensive Bluetooth scanners work with phone apps. That is useful, but a code is a clue, not a verdict. A lean condition code, for example, can come from a vacuum leak, a weak fuel pump, a dirty airflow sensor or a leaking intake gasket. Replacing the part named in a code description is one of the most common ways drivers spend money without fixing anything.
A real diagnosis usually includes:
- Reading all stored and pending codes and the freeze-frame data, which records conditions when the fault was set.
- Checking for technical service bulletins and known issues on the model.
- Reviewing live data, such as fuel trims, sensor readings and misfire counts.
- Testing the suspected circuit or component with a meter, smoke machine, fuel pressure gauge or other tools.
- Repairing the cause, clearing the codes and confirming the fix with a test drive.
Diagnostic fees commonly run from about $100 to $200 for the first hour, and some technicians credit the fee toward the repair. Prices vary, so ask first. A mobile technician can do much of this work where the car sits, as covered in what a mobile mechanic can fix.
Codes and what they mean in plain terms
| Code range | System | Example |
|---|---|---|
| P0100 to P0199 | Fuel and air metering | P0171: system too lean, bank 1 |
| P0200 to P0299 | Fuel injectors | Injector circuit faults |
| P0300 to P0399 | Ignition and misfire | P0300: random misfire; P0301: cylinder 1 misfire |
| P0400 to P0499 | Emissions controls | P0420: catalyst efficiency below threshold, bank 1; P0455: large evaporative leak |
| P0500 to P0599 | Speed and idle control | Vehicle speed sensor faults |
| P0700 to P0999 | Transmission | Often linked to transmission control faults |
Codes starting with P1 are manufacturer-specific and need model information to interpret. Transmission codes deserve particular attention; see transmission warning signs.
Clearing the light is not fixing the car
A scan tool can clear codes and turn the light off, and disconnecting the battery often does the same. The light will come back if the fault remains. Clearing codes also resets the readiness monitors, the self-tests the computer runs over several drive cycles. Until those monitors complete, the vehicle may not be ready for an emissions test. This catches out people who clear a light the day before an inspection. The Georgia emissions testing guide explains how the inspection treats the light and the monitors.
When a check engine light comes with symptoms
- Rough running, shaking or loss of power: likely a misfire. Reduce load and get it checked.
- Rotten-egg smell: can indicate a catalytic converter problem or a rich mixture.
- Overheating: treat the temperature problem first, as described in what to do when your car overheats.
- Harsh or erratic shifting: the transmission may be in a protective mode.
Frequently Asked Questions
Is it safe to drive with the check engine light on?
If the light is steady and the car drives normally, it is usually safe to drive for a short time while you arrange a check. If the light is flashing or the car runs badly, reduce speed and have it inspected promptly to avoid damage.
Can a loose gas cap turn on the check engine light?
Yes. A loose or faulty cap can set an evaporative system leak code. Tighten it until it clicks; the light may take several days of normal driving to turn off.
Where can I get my check engine light code read?
Many auto parts stores read codes for free, and inexpensive scanners work with phone apps. A technician can go further with live data and circuit testing to find the actual cause.
Will my car pass emissions with the check engine light on?
No. In the metro Atlanta counties that require testing, an illuminated check engine light causes a failure. Fix the fault and allow the readiness monitors to complete before testing.
Why did the light come back after I cleared it?
Clearing codes does not fix the underlying fault. When the computer detects the problem again, it stores the code and turns the light back on.