Check Engine Light
What the light means, what the codes mean, and when to stop driving.
9 answers
- Not urgent
Why is my "check fuel cap" light on?
A "check fuel cap" (or "check gas cap") message means the car's evaporative emissions (EVAP) system found a fuel vapor leak, and the most likely cause is a gas cap that's loose, missing or has a worn seal. Tighten the cap until it clicks, replace it if the seal is cracked, and expect the light to take a few days of normal driving to go off.
- Not urgent
What does code P0442 mean?
P0442 means the evaporative emissions (EVAP) system, which holds gasoline vapor from the tank, found a small leak. Rick's Free Auto Repair Advice calls a loose or faulty gas cap by far the most common cause, followed by cracked EVAP hoses and a purge or vent valve that doesn't seal fully.
- Not urgent
What does code P0455 mean?
P0455 means the evaporative emissions (EVAP) system, which traps gasoline vapor from the fuel tank, found a large leak, roughly a hole 0.04 inch across or bigger. The most common cause is a loose, missing or worn gas cap, so check that first; after that the usual suspects are a stuck vent or purge valve, a cracked EVAP hose or a damaged charcoal canister.
- Not urgent
What does code P0456 mean?
P0456 means the evaporative emissions (EVAP) system, which keeps gasoline vapor from escaping the fuel tank, found a very small leak during its self-test. A loose or worn gas cap is the most common cause, so check it first; after that the usual suspects are a cracked EVAP hose, a purge or vent valve that doesn't seal, or a damaged charcoal canister.
- Get it checked soon
What does code P0171 mean?
P0171 means "System Too Lean (Bank 1)" — the engine computer is adding as much fuel as it's allowed to and the mixture is still running lean, meaning too much air or too little fuel. The most common causes are a vacuum leak (a cracked intake boot, hose or PCV line), a dirty or failing mass airflow (MAF) sensor, and low fuel pressure, so start there rather than replacing oxygen sensors.
- Act now
What does code P0300 mean?
P0300 means "Random/Multiple Cylinder Misfire Detected" — the engine computer sees misfires that aren't tied to one cylinder. The usual suspects are worn spark plugs or ignition coils, a vacuum leak, low fuel pressure or a bad mass airflow sensor, and it's best not to keep driving on it, because a misfire can damage the engine and the catalytic converter.
- Act now
Why is my check engine light flashing?
A flashing (blinking) check engine light almost always means the engine is misfiring badly enough to send unburned fuel into the exhaust, which can overheat and destroy the catalytic converter. Ease off the gas, get off the road when it's safe, and don't keep driving it normally until the misfire is diagnosed.
- Get it checked soon
Is it safe to drive with the check engine light on?
If the check engine light is solid and the car drives normally, it's usually safe to keep driving short-term, but get the code read soon because small faults can turn into expensive ones. If the light is flashing, or the car runs rough, overheats, or shows low oil pressure, stop driving and get it looked at.
- Get it checked soon
What does code P0420 mean?
P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)" — the engine computer has decided the catalytic converter on bank 1 isn't cleaning the exhaust as well as it should. The converter itself is often worn out, but an exhaust leak, a bad downstream oxygen sensor, or an engine problem that damaged the converter can set the same code.