Faulty or drifting EGR temperature sensor (A or B); Damaged wiring, poor connector contact or corrosion in the sensor circuits; Sooted or sticking EGR valve causing abnormal or no flow, so temperatures do not match expectations; Blocked or leaking EGR cooler or bypass flap fault on engines that have them; Sensor contaminated with soot or coolant residue; Exhaust leak near the sensors or EGR pipework affecting readings; Control unit fault or outdated software, where a known issue is documented for the model.

P040F

EGR Temperature Sensor A/B Correlation.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
0F: FaultThe specific fault in that area

What it means

P040F means the engine control unit has compared the two exhaust gas recirculation (EGR) temperature sensors, A and B, and found their readings do not agree as expected. This is usually checked when the EGR flow is active, or after a cold start or soak, when both sensors should read similarly or follow the expected temperature difference. The exact sensor positions (for example before and after the EGR cooler) vary by engine, so check the model-specific information.

Symptoms

Engine warning light on
Reduced power or the engine going into a limp mode on some vehicles
Rough idle or hesitation when EGR is operating
Increased exhaust smoke or higher fuel consumption
Other EGR codes stored alongside, such as flow or cooler efficiency faults

Possible causes

Faulty or drifting EGR temperature sensor (A or B)likely
Damaged wiring, poor connector contact or corrosion in the sensor circuitslikely
Sooted or sticking EGR valve causing abnormal or no flow, so temperatures do not match expectationspossible
Blocked or leaking EGR cooler or bypass flap fault on engines that have thempossible
Sensor contaminated with soot or coolant residuepossible
Exhaust leak near the sensors or EGR pipework affecting readingsrare
Control unit fault or outdated software, where a known issue is documented for the modelrare

How to diagnose

Read all stored and pending codes and freeze frame data, noting engine temperature, load and EGR commanded position when the fault set
Inspect the wiring, connectors and sensor mounting for chafing, heat damage, corrosion, water ingress or loose pins
Compare live data from both EGR temperature sensors on a cold engine after a long soak; they should read close to ambient and to each other
Run the engine and watch both readings with EGR commanded open and closed, checking they respond plausibly for your engine's layout
Check the sensor circuits for reference voltage, ground and signal, and measure sensor resistance against the manufacturer's temperature table
Inspect the EGR valve for sticking, carbon build-up and correct movement using a bidirectional test if available
Check the EGR cooler and bypass for blockage, leaks or coolant loss, and look for exhaust leaks around the pipework
Check for relevant manufacturer service bulletins or software updates for the specific model and engine

How to fix

Replace the faulty EGR temperature sensor and clear the code once the live readings agree
Repair or replace damaged wiring, terminals or connectors in the sensor circuits
Clean or replace a sticking or sooted EGR valve in line with the manufacturer's procedure
Replace or repair a blocked or leaking EGR cooler or a faulty bypass actuator
Repair any exhaust leak near the sensors or EGR pipework
Apply an updated control unit calibration only where the manufacturer lists it as a fix for this fault

Can I keep driving?

Medium severity

The car can usually be driven short-term, but get it checked soon as EGR faults can cause poor running, higher emissions and possible limp mode. Stop and seek help if power drops sharply, coolant is being lost or the engine runs badly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.