Guides
Plain-English guides to OBD-II: how the check engine system works, how to read a code yourself, how codes relate to each other and what to check after a repair.
Check engine light
What the light is telling you, when it is safe to keep driving, and what to do next.
Check engine light How to Clear a Check Engine Light (and When Not To)
Clear a check engine light with a scan tool, by fixing the fault, or by disconnecting the battery. Why clearing before inspection fails, and when not to clear.
Check engine light Gas Cap Check Engine Light: Codes, Fix and Reset Time
A loose gas cap can set the check engine light with codes like P0455 or P0457. How to check the cap, and why the light takes several drives to go off.
Codes explained
How families of codes relate to each other, and why one problem can set several codes.
Codes explained Pending vs Stored vs Permanent Codes: What's the Difference?
Pending codes are faults seen once, stored codes are confirmed, and permanent codes survive a scan tool clear. How each sets, clears and affects smog checks.
Codes explained What Is a DTC? How to Read a Diagnostic Trouble Code
A DTC is a five-character code your car's computer stores when a self-test fails. What P, B, C and U mean, P0 vs P1 codes, and extras like -00 or U0100-00.