P040F: EGR Temperature Sensor A/B Correlation

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.

Medium severity. Can I keep driving?You can normally drive in moderation, but get it repaired soon: if the valve is stuck open, the engine may idle very poorly.

Symptoms

Check engine light on, may come and go
No change in driving

Most likely causes

One sensor drifting
Contact resistance at one sensor's connector
Connectors swapped after a repair
Unrecognised heat source (auxiliary heater)

What to check, in order

Start with the simplest. You can tick off each step.

Check the connector and any vacuum hoses
Compare both signals at the same time in live data and check any recent installation work.
Use live data to compare the commanded opening with the actual opening
Clean the valve and passages if they are clogged with carbon
Test the actuator with the scanner
Replace the valve if it still does not follow the command after cleaning

Good to know

Repair difficulty: easy
It affects emissions.

What each character of P040F means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
0F: Specific fault in that system

Frequently asked questions

Can I keep driving with code P040F?

You can normally drive in moderation, but get it repaired soon: if the valve is stuck open, the engine may idle very poorly.

What causes code P040F?

The most common causes are: one sensor drifting; contact resistance at one sensor's connector; connectors swapped after a repair.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.