P040A

What it means

P040A is the generic SAE J2012 code for a fault in the EGR Temperature Sensor 'A' circuit. The engine control unit monitors the signal voltage from the exhaust gas recirculation temperature sensor and has seen a reading or behaviour that is not as expected for the circuit. Which exact condition sets the code (open circuit, short, or implausible signal) depends on the vehicle.

Symptoms

• Engine management light on
• Possible slightly rough idle or hesitation, depending on how the EGR system is affected
• Reduced fuel economy in some cases
• Failed emissions test
• Other EGR-related codes stored alongside it

Possible causes

• Faulty EGR temperature sensor (internal open or short circuit, or sooting/contamination)likely
• Damaged, chafed or corroded wiring to the sensor, including shorts to earth or voltagelikely
• Poor connection at the sensor or control unit connector (corrosion, loose or spread terminals)likely
• Heat damage to the sensor harness or connector close to the exhaust or EGR valvepossible
• Sensor reading implausible due to a blocked, stuck or leaking EGR valve or heavy carbon build-up in the EGR passagespossible
• Fault in the engine control unit (sensor supply or input circuit)rare

How to diagnose

1. Read all stored codes and freeze frame data, noting engine temperature, load and speed when the code set, and check for other EGR or sensor supply codes
2. Visually inspect the sensor, harness and connector for heat damage, chafing, corrosion, oil or coolant contamination
3. Check live data for the EGR temperature reading with the engine cold and compare it with intake or coolant temperature; it should be similar, then rise as EGR flows
4. Disconnect the sensor and check its resistance or signal against the manufacturer's specification at a known temperature, as the sensor type varies by model
5. Check the sensor wiring for continuity, short to earth and short to voltage, and verify the reference supply and earth at the connector
6. Inspect the EGR valve and passages for sooting, sticking or leaks, and check that EGR flow changes the temperature reading as expected
7. Clear the code and road test to see if it returns, confirming the fix

How to fix

• Replace the EGR temperature sensor if it is out of specification or contaminated
• Repair or replace damaged wiring and secure the harness away from heat sources
• Clean or replace corroded or damaged connectors and terminals
• Clean or replace a sticking or clogged EGR valve and clear the passages, following the manufacturer's procedure
• Check for a software update or revised calibration only if the manufacturer lists one for this fault, and have the control unit tested if everything else checks out

Can I keep driving?

Nearby codes

• P0409GenericEGR Sensor A Circuit
• P040BGenericEGR Temperature Sensor A Circuit Range/Performance