What Does P040F Mean?
The ECM has detected that the EGR temperature sensors A and B are not correlating with each other. These two sensors, positioned at different points in the EGR system, should show a logical temperature relationship based on exhaust gas flow direction and cooling. When their readings diverge beyond the acceptable threshold, this code is set. The driver will not notice drivability symptoms, but EGR flow verification is compromised.
Common Causes
35%
One of the two EGR temperature sensors has drifted out of calibration
25%
Carbon buildup on one sensor probe more than the other, insulating it from gas flow
15%
Wiring issue on one sensor circuit causing a slight offset in readings
15%
EGR passage partially blocked between the two sensor locations
10%
Incorrect sensor installed at one location (wrong part number)
Diagnostic Steps
1
Compare both EGR temperature sensor PIDs at cold start — they should both read near ambient temperature (within 5-10F of each other). A large difference at cold start indicates one sensor has drifted.
2
Command the EGR valve open and monitor both sensors' response — both should show temperature increases, though at different rates depending on their position. A sensor that does not respond may be carbon-fouled or failed.
3
Remove both sensors and test their resistance at room temperature and at an elevated temperature (use a heat gun or warm water) — compare to the manufacturer's resistance chart. The one that deviates is the faulty sensor.
4
Inspect the EGR passages at both sensor locations for differential carbon buildup that could insulate one sensor from exhaust gas flow.
5
Verify both sensors are the correct part number — some systems use sensors with different calibration curves at different positions.
Estimated Repair Cost
$60 - $300
Parts + labor, varies by vehicle and location
The ECM has detected that the EGR temperature sensors A and B are not correlating with each other. These two sensors, positioned at different points in the EGR system, should show a logical temperature relationship based on exhaust gas flow direction and cooling. When their readings diverge beyond t...
The most common cause of P040F (Exhaust Gas Recirculation Temperature Sensor "A"/"B" Correlation) is: One of the two EGR temperature sensors has drifted out of calibration
Typical repair costs for P040F range from $60 to $300, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive. This is a correlation fault between two monitoring sensors. No effect on engine performance or safety. Will affect emissions readiness monitors and testing.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P040F to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions Control / EGR System
Difficulty
Type
Generic (SAE)
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