Powertrain, generic · Auxiliary emission controls
P040F EGR Temperature Sensor A/B Correlation
Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo
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.
Likely causes
One sensor drifting
Contact resistance at one sensor's connector
Connectors swapped after a repair
Unrecognised heat source (auxiliary heater)
Symptoms
- Check engine light on, may come and go
- No change in driving
Repair estimate
DifficultyEasyDIY possible
Parts and labour€50–300
Time0.5–2 h
Typical range across vehicles; the real cost depends on what the testing finds.
Parts involved
egr temperature sensor aegr temperature sensor bsensor wiringengine control module
Related codes
References: gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.