OBD-II & Trouble Codes: questions & answers
What OBD-II trouble codes mean, how monitors and freeze frame work, and how to diagnose common codes the right way.
Common P-Codes Explained
- What does P0300 mean and where do I start?
- How do I diagnose a single-cylinder misfire code like P0302?
- What causes P0171 system too lean bank 1?
- What does P0420 mean and is it always a bad catalytic converter?
- What causes P0128 coolant thermostat below regulating temperature?
- How do I find the leak behind a P0442 or P0455 EVAP code?
OBD-II Basics
- What does a diagnostic trouble code actually tell me?
- What do the letters P, B, C and U mean at the start of a code?
- Where is the OBD-II port and does every car have one?
- What is the difference between a pending code and a confirmed code?
- What is a permanent diagnostic trouble code?
- Is it okay to drive with the check engine light on?
Reading and Clearing Codes
Freeze Frame and Readiness
U-Codes and Network Faults
Diagnostic Strategy
- What is a good step-by-step diagnostic process for a check engine light?
- What is mode 6 data and why is it useful?
- Should I replace the part a code names?
- How do I diagnose an intermittent problem that will not set a code when the car is in the shop?
- Why should I check technical service bulletins before diagnosing?
Diesel Emissions Codes
Emissions Inspection Readiness
- Why did a car fail inspection even though the check engine light is off?
- How long does it take readiness monitors to reset after clearing codes?
- What is the difference between continuous and non-continuous monitors?
- Why won't the EVAP monitor complete?
- Can a vehicle pass an emissions inspection with incomplete monitors?