Type the code from a scanner, like P0420, and see what it means in plain English, how serious it usually is, and what to do about it if the car is for sale.
Enter one code, or several separated by commas or spaces.
Common codes
How to read a code
PSystem0Generic or maker4Subsystem20Specific fault
The letter is the system: P for powertrain (engine and transmission), B for body, C for chassis (brakes and steering), U for the network that links the car’s computers.
The first digit says who defined it. 0 is a standard code that means the same thing on every car. 1 is specific to the manufacturer.
The rest narrows it down. P03xx codes are ignition and misfires, P04xx emissions controls, P07xx the transmission.
A code tells you where the car found a problem, not always which part failed. Treat it as the start of a diagnosis.
Browse every code in our table
The standard codes we explain, with how serious each one usually is. Tap a code to open it.
Code
What it is
Seriousness
Before you trust a clean scan
How to tell if someone cleared the codes before you arrived.
Check the light with the key on. With the engine off and the key on, the check engine light should glow, then go out once the engine starts. If it never lights up at all, the bulb may have been disabled.
Read the codes, including pending ones. A pending code is a problem the car has seen once and is still confirming. Pending codes can show up before the light comes on.
Read the readiness monitors. These are the car’s own self-tests. When codes are cleared or the battery is disconnected, they all reset to “not ready” and take days of mixed driving to complete. Several monitors showing not ready on a car that is supposedly driven every day is a red flag.
Ask directly. “Has the check engine light been on recently, or has the battery been disconnected?” A vague answer plus incomplete monitors is your cue to walk, or to have a mechanic look before you pay.
A car with its check engine light on, or with too many monitors not ready, will not pass a Utah emissions test in the counties that test. See whether your county tests.
Found a code on a car you like?
We can help you get it diagnosed, price the repair, and use it to negotiate, or tell you when it is time to walk away.
How this works. Code names follow the SAE J2012 standard, which defines the generic codes that mean the same thing on every car sold in the US since 1996. The plain-English meaning, the usual causes and the seriousness rating are our own guidance from common repair experience, not a diagnosis. The same code can be a cheap sensor on one car and a major repair on another, so have a mechanic confirm the cause before you pay for a fix or a car. Codes that start P1, B1, C1 or U1 are defined by each manufacturer and are not in this table. Checked October 2026 against: