Faulty EGR valve position sensor or integrated sensor inside the EGR valve assembly; Damaged, chafed or shorted wiring to the sensor signal or reference supply, including a short to earth; Corroded, loose or contaminated connector terminals at the EGR valve or sensor; Open circuit in the 5 volt reference supply feed to the sensor; EGR valve sticking or heavily carbon-fouled, giving an implausible sensor reading; Faulty engine control unit or its internal sensor supply or input circuit.

P0405

EGR Sensor A Circuit Low.

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
05: FaultThe specific fault in that area

What it means

P0405 is the generic code for a low signal from the exhaust gas recirculation (EGR) sensor 'A' circuit. The engine control unit monitors the sensor's voltage, which normally reflects EGR valve position or EGR flow. It saw the voltage fall below the expected minimum range, which points to a short to earth, an open circuit or a faulty sensor. Where the sensor sits and what it measures (valve position, pressure or flow) varies by model and engine.

Symptoms

Engine warning light on
Rough or unstable idle
Hesitation or poor response when accelerating
Engine stalling or surging at low speed
Slightly higher fuel consumption
Failed emissions test

Possible causes

Faulty EGR valve position sensor or integrated sensor inside the EGR valve assemblylikely
Damaged, chafed or shorted wiring to the sensor signal or reference supply, including a short to earthlikely
Corroded, loose or contaminated connector terminals at the EGR valve or sensorlikely
Open circuit in the 5 volt reference supply feed to the sensorpossible
EGR valve sticking or heavily carbon-fouled, giving an implausible sensor readingpossible
Faulty engine control unit or its internal sensor supply or input circuitrare

How to diagnose

Read the stored and pending codes and freeze frame data, noting engine temperature, speed and load when the fault set, and check for related EGR or sensor supply codes.
Inspect the EGR valve or sensor connector and wiring for damage, heat, chafing, water ingress and corrosion.
Check live data for the EGR sensor voltage or position reading and compare it with the commanded EGR position, using the manufacturer's specification for your model.
Wiggle the harness and connector with the engine running and watch for sudden changes in the sensor reading.
With the ignition on, measure the reference supply and earth at the sensor connector against the wiring diagram for that engine.
Test the signal wire for continuity and for a short to earth between the sensor and the control unit.
Inspect the EGR valve for carbon build-up or a stuck mechanism, and check it moves freely if the design allows.
If wiring and supply are good, test or substitute the sensor or valve assembly following the manufacturer's procedure, as the sensor may not be sold separately.

How to fix

Repair or replace damaged wiring and clean or replace corroded connector terminals.
Replace the EGR valve or sensor assembly if the sensor output is faulty.
Restore the sensor reference supply or earth if an open circuit or poor connection is found.
Clean or replace a carbon-fouled or sticking EGR valve, following the manufacturer's guidance for your model.
Clear the codes after repair, then road test and confirm the code does not return and the readings are plausible.
Have the control unit diagnosed and repaired or replaced only after all wiring, supply and sensor faults have been ruled out.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but book it in soon because poor running, increased emissions and further EGR faults may develop. If it stalls, surges or runs very roughly, limit driving until it is checked.

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