What it means
EGR temperature sensors A and B disagree by more than the allowed tolerance over a drive cycle. EGR mixes inert exhaust into intake to lower combustion peak temperature — too little raises NOx, too much causes misfires and rough running.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
EGR temperature sensors A and B measure temperatures at separate points in the EGR system so the control module can check how exhaust heat changes through the system. P040F sets when the difference between their readings exceeds the allowed correlation range.
Common causes
Most likely first.
- EGR temperature sensor A or B drifting or faulty
- Open, shorted, or damaged sensor wiring
- Corroded or loose connector
- EGR cooler or flow problem causing an unexpected temperature difference
Symptoms
- Check engine light on
- Pinging or knocking under load
- Power loss and hesitation
- Possible rough idle
- Failed emissions inspection (NOx)
How to diagnose it
- Record freeze-frame data and compare live EGR temperature sensor A and B readings from a cold start and as the system warms.
- Inspect both sensors' connectors and wiring for corrosion, loose pins, heat damage, or chafing that could skew one reading.
- Check each sensor's signal, reference, and ground circuits against the wiring diagram, looking for an open or short affecting either reading.
- Compare the A-to-B temperature change with commanded EGR operation and inspect EGR flow and cooler operation for a cause of an unexpected temperature difference.
- Test sensors A and B individually against the manufacturer's specification to identify drift or a circuit fault.
Common fixes
- Clean and secure a loose or corroded connector at sensor A or B.
- Repair damaged or shorted wiring affecting either temperature sensor.
- Replace the sensor that fails the manufacturer's specification.
- Repair an EGR cooler or flow fault causing an abnormal temperature difference.
Can I keep driving?
If the engine runs smoothly, a short drive may be possible, but avoid heavy loads if knocking, hesitation, or power loss occurs. The disagreement can impair EGR control and increase emissions; stop if symptoms make the vehicle unsafe.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.