What it means
EGR temperature sensor A below lower plausibility — short to ground. 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 sensor A measures exhaust gas temperature in the EGR path for the control module. P040C sets when its signal is lower than the circuit's expected range, commonly because the signal is shorted to ground.
Common causes
Most likely first.
- Short to ground
- Connector pins corroded - high contact resistance
- Harness chafing through insulation
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 check sensor A's live temperature reading against other temperature readings to confirm the low signal.
- Inspect sensor A's harness and connector for chafing, exposed wire, corrosion, or a terminal contacting ground.
- Use the wiring diagram to check the signal circuit for a short to ground and inspect the sensor's reference and ground connections.
- If the wiring is intact, monitor sensor A while the EGR path warms to see whether its reading changes plausibly.
- Test sensor A and the circuit against the manufacturer's specification to determine whether the sensor itself has failed.
Common fixes
- Repair chafed or damaged sensor A wiring that is shorting to ground.
- Clean and secure corroded or loose connector terminals.
- Replace EGR temperature sensor A if it fails the manufacturer's specification.
- Correct an EGR flow or cooler fault if it is contributing to implausible temperature readings.
Can I keep driving?
A short drive may be possible if the engine runs normally, but avoid heavy loads if knocking or hesitation develops. A low sensor A signal can interfere with EGR temperature control, raise emissions, and contribute to poor running.
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.