Diagnostic trouble codes (DTCs) are short codes your car stores when it detects a problem. Codes starting with P are powertrain (engine and transmission); B is body, C is chassis, and U is network communication.
One code, several possible causes
Take P0420, "catalyst system efficiency below threshold." The catalytic converter could be worn, but the same code can come from a faulty oxygen sensor, an exhaust leak, or an engine problem upstream. Replacing the converter without testing can be an expensive guess.
What to ask the shop
- What tests will you run to confirm the cause?
- What's the diagnostic fee, what does it cover, and is it credited toward the repair?
- Can I see the test results or readings that point to this part?
- Are there any other stored or pending codes?
Bring every code (including pending ones) to your ShopCheck request so shops can quote an accurate diagnosis.