P040C

What it means

P040C is the generic code for EGR Temperature Sensor A Circuit Low. The engine control unit saw the signal voltage from the exhaust gas recirculation temperature sensor fall below the expected range, which usually points to a short to earth in the circuit or a sensor reading that is not plausible. Sensor position and the exact trigger limits vary by model and engine.

Symptoms

• Engine warning light on
• Car may go into a reduced-power or limp mode on some models
• Rough idle or slight hesitation if EGR operation is restricted
• Increased exhaust smoke or higher emissions on some engines
• Other EGR-related codes stored alongside it
• Failed emissions test at MOT

Possible causes

• Faulty EGR temperature sensor (internal short or drift)likely
• Signal wire chafed and shorted to earthlikely
• Damaged, corroded or water-contaminated connector or terminalslikely
• Heat damage to the wiring or sensor near the EGR cooler or exhaustpossible
• Sensor reference supply or earth fault in the wiringpossible
• Fault inside the engine control unitrare

How to diagnose

1. Read all stored codes and freeze frame data, noting engine temperature and load when the fault set, then check for related EGR codes
2. Inspect the sensor, its wiring and connector for chafing, heat damage, corrosion, water ingress and loose terminals
3. Check live data for the EGR temperature reading with the engine cold; it should be close to ambient or coolant temperature and not fixed at an extreme low value
4. Watch the reading while the engine warms up and while the EGR valve operates, checking it rises plausibly and does not jump or stick
5. Unplug the sensor and check live data and the circuit voltage against the manufacturer's specification, as the expected values depend on the model
6. Test the sensor resistance at different temperatures against the manufacturer's data
7. Check the wiring for continuity and for a short to earth between the sensor and the control unit
8. Clear the code and road test to see if it returns, only after repairs are complete

How to fix

• Replace the EGR temperature sensor if its readings are out of specification
• Repair or replace damaged wiring where it is shorted to earth or chafed
• Clean, repair or replace corroded or damaged connector terminals and seal out moisture
• Secure and reroute wiring away from hot exhaust components, using heat protection where needed
• Repair the supply or earth fault found in the sensor circuit
• Have the control unit checked or replaced only after all wiring and sensor faults are ruled out

Can I keep driving?

Nearby codes

• P040BGenericEGR Temperature Sensor A Circuit Range/Performance
• P040DGenericEGR Temperature Sensor A Circuit High