What is the difference between P0404 and P0405?

P0404 is a position range/performance fault — the valve cannot reach the commanded position. P0405 is an electrical circuit fault — low voltage on the signal wire. P0404 is often mechanical; P0405 is always electrical.

Can P0405 be caused by a wiring short?

Yes — this is one of the most common causes. Damaged insulation on the signal wire contacting earthed metalwork creates a path to earth that pulls the sensor signal voltage low.

How do I test the EGR position sensor?

With the connector plugged in and ignition on, back-probe the signal wire while manually moving the EGR valve (if accessible). Voltage should change as the valve moves. A fixed low reading confirms a failed sensor or short circuit.

Will a new EGR valve fix P0405?

Only if the fault is in the integrated sensor or connector. If the fault is a wiring short, replacing the valve will not help — fix the wiring first.

Does P0405 affect my fuel consumption?

Yes. Incorrect EGR feedback causes the ECU to run the EGR system incorrectly, affecting combustion efficiency and emissions. Fixing P0405 restores correct EGR operation.

Is P0405 expensive to repair?

A replacement EGR valve typically costs £80–£300. If the fault is purely a wiring short, the repair cost can be much lower — just the cost of wiring repair materials and labour time.

P0405 — EGR Sensor A Circuit Low

What Is P0405?

P0405 means the ECU has detected a low voltage on the EGR valve position sensor circuit. The position sensor provides feedback on EGR valve opening percentage — a low-voltage signal means the sensor circuit is reading below the minimum expected threshold, indicating a short to earth, a failed sensor, or a wiring fault.

Unlike P0404 (which is a position mismatch/performance fault), P0405 is a purely electrical fault in the sensor signal circuit. The EGR valve may be mechanically functional, but without correct position feedback the ECU cannot control it accurately.

Common Symptoms

Engine management light on
EGR valve operating incorrectly (stuck open or closed)
Rough idle or stumbling
Increased smoke on diesel
Poor fuel economy
Failed emissions test
Possible limp mode

Common Causes

Short to earth on the EGR position sensor signal wire

Failed EGR position sensor — internally shorted, producing a low output voltage

Corroded connector at the EGR valve with low-resistance path to earth

Damaged wiring insulation breached and contacting earthed metalwork

Failed 5V reference supply to the sensor from the ECU

ECU signal input fault — rare

How to Diagnose P0405

Disconnect the EGR Connector and Recheck

Unplug the EGR valve connector and clear the fault. If P0405 reappears with the connector unplugged, the short is in the wiring loom. If it clears, the fault is in the valve assembly itself (sensor or connector).

Check Reference Voltage

With ignition on and connector plugged in, back-probe the reference voltage pin. Should be 5V. No voltage here means the ECU reference supply has failed — check other sensors on the same reference circuit.

Test Signal Wire for Short

With connector unplugged, measure resistance between the signal wire and earth. Below 1 ohm indicates a direct short to earth in the wiring. Trace the loom for damage.

Inspect the Connector

EGR connectors on diesels can be contaminated with oily carbon deposits. Clean with electrical contact cleaner. Inspect for bent or corroded pins that may be bridging the signal pin to an earthed body pin.

Replace the EGR Valve

If the connector is clean and the wiring is intact but P0405 persists, the position sensor integrated into the EGR valve has failed. Replace the complete valve assembly.

Check Related Sensors

If the ECU 5V reference supply has failed (no voltage on reference pin), other sensors on the same reference circuit (MAP sensor, throttle position sensor) will also give faults. Check for additional related codes.

5V Reference CircuitMany sensors share a common 5V reference supply from the ECU. If P0405 appears alongside MAP sensor, TPS, or other sensor circuit faults, the ECU 5V reference output may have failed rather than any individual sensor.

Verdict

Disconnect the connector and recheck — this instantly identifies whether the fault is in the wiring or the valve. A short to earth in the signal wire or a failed integrated sensor are the two most likely causes.