P040F: Exhaust Gas Recirculation Temperature Sensor A/B Correlation
What to do
Leaving the car overnight genuinely helps here. This fault is diagnosed from stone cold, when both sensors should read the same as the outside air — a warm engine tells the garage much less.
Ask whether the EGR cooler has been checked, not just the sensors. These two sensors sit either side of the cooler and measure whether it is working, so a blocked or bypassed cooler produces this code with two perfectly good sensors.
Mention any white smoke from the exhaust or coolant disappearing without a visible leak. An EGR cooler can split internally and leak coolant into the engine, which is a far more serious finding and worth flagging early.
Frequent short journeys accelerate soot build-up in the whole EGR system, so occasional longer runs help.
What this code means
The two EGR temperature sensors disagree by more than the module allows.
Two sensors on an EGR path are usually placed either side of the cooler, and that arrangement is the whole point of this code. The difference between them is the cooler's effectiveness, measured directly — hot gas in, cooled gas out. The module does not need to guess whether the cooler is working; it can watch it happen.
So a correlation fault means one of two quite different things.
- One sensor has drifted or is soot-fouled, so the measured difference is wrong while the cooler is fine.
- The difference genuinely is not there — the cooler is blocked, bypassed, or its coolant flow has stopped, so gas leaves at nearly the temperature it entered.
The cold soak separates them without a road test. After the vehicle has stood overnight both sensors must read close to ambient and close to each other. If they disagree then, a sensor is at fault; if they agree cold and fail to diverge when running, the cooler is.
That single check answers the question the code cannot, and it costs nothing but leaving the car overnight.
A cooler that is not cooling is worth finding for a second reason. An internally split EGR cooler leaks coolant into the intake, which shows as white exhaust smoke and coolant disappearing with no visible leak — a much larger problem than a sensor.
Likely causes
Start here
- Soot-fouled EGR cooler · moderate
- Soot contamination in the sensor port · moderate
- Faulty EGR temperature sensor · moderate
Less common
- Seized EGR cooler bypass flap · moderate
- High resistance in sensor wiring · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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