P040FEGR Temperature Sensor A/B Correlation
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The two EGR temperature sensors measure the gas temperature in the EGR path, before and after the cooler – which is A and which is B depends on the maker. P040F compares sensors A and B: after a long stop (GM: at least 8 hours) both must read about ambient temperature. If they differ by more than the calibrated limit at start-up (GM Duramax LML table, at least 20 °C, with exclusions for block heaters), the code is set, MIL after two trips. It does not say which sensor is wrong – compare both with intake air and coolant temperature. Ford diesels cover the same goal with P041B.
- What you may notice
- Check engine light on, may come and go
- No change in driving
- Likely causes
- LikelyOne sensor drifting
- PossibleContact resistance at one sensor's connector
- RareConnectors swapped after a repair
- RareUnrecognised heat source (auxiliary heater)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €50 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.