What some of the most frequent warning codes mean, in plain English, and how urgent each one is.
Every code starts with a letter for the system involved: P for engine and transmission, B for body, C for chassis and U for communication between the car's computers. Codes with a 0 as the second character are standard on every car; a 1 means the code is specific to your car's manufacturer.
The system that traps gasoline vapor has a large leak. The most common cause is a gas cap that's loose, missing or worn. Tighten the cap until it clicks; the light often clears after a few days of driving.
A small vapor leak was found, often from a worn gas cap seal or a cracked hose. It won't hurt the engine, but it can cause a failed emissions inspection.
The engine is taking longer than expected to reach normal temperature, usually because the thermostat is stuck open. Expect weaker cabin heat and slightly worse fuel economy. Have it replaced when convenient.
The part that cleans your exhaust isn't doing its job fully. It's usually safe to drive for now, but it can cause a failed emissions inspection, and a bad oxygen sensor can trigger it too. Have it diagnosed in the next few weeks.
The engine is getting too much air or too little fuel. Common causes are a vacuum leak, a dirty airflow sensor or a weak fuel pump. Left alone, it can cause rough running and engine damage over time.
An oxygen sensor's built-in heater isn't working, so the engine computer gets less accurate readings while the engine warms up. Fuel economy and emissions can suffer until the sensor is replaced.
A system that lowers engine combustion temperatures isn't flowing enough, often because of carbon buildup. It can cause engine knock (pinging) and a failed emissions test.
The engine is idling faster than it should, often from a vacuum leak or a dirty throttle body. It's mostly an annoyance, but get it checked so a small leak doesn't grow.
One or more cylinders isn't firing properly. If the check engine light is flashing, pull over when it's safe and stop driving: unburned fuel can overheat and destroy the catalytic converter.
Cylinder 1 isn't firing properly, often because of a worn spark plug or a failing ignition coil. Avoid driving, especially if the light is flashing or the engine shakes. Codes P0302 to P0306 mean the same thing for other cylinders.
The engine is hotter than it should be. Stop driving as soon as it's safe and turn the engine off. Driving an overheating engine can warp or crack major engine parts.
The sensor that watches your oil pressure has a fault, so the car can't warn you if oil pressure drops. Check your oil level, and don't drive until it's been checked: low oil pressure can destroy an engine within minutes.
These are general explanations. The same code can have different causes on different vehicles, and a code shows where a problem was detected, not always what caused it. For a diagnosis and repair, see a qualified mechanic.