Faulty EGR sensor B (position, pressure or temperature type depending on model); Damaged, chafed or shorted-to-earth signal wire in the sensor circuit; Corroded, loose or contaminated connector at the sensor or control unit; Open circuit or high resistance in the sensor reference supply or earth; Carbon build-up in the EGR valve or passages affecting the sensor or its readings; Faulty EGR valve assembly with integrated sensor; Fault within the engine control unit.

P0407

EGR Sensor B 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
07: FaultThe specific fault in that area

What it means

P0407 means the engine control unit has detected a low voltage signal from the exhaust gas recirculation (EGR) sensor 'B' circuit. The signal has fallen below the expected range, which usually points to a short to earth, an open circuit or a failed sensor. Which sensor counts as 'B' (for example a flow, position or temperature sensor) depends on the vehicle, so check the manufacturer's information for your model.

Symptoms

Engine warning light on
Rough or unstable idle
Hesitation or poor throttle response
Slightly reduced fuel economy
Possible pinging or knocking under load
Occasional stalling at idle on some engines

Possible causes

Faulty EGR sensor B (position, pressure or temperature type depending on model)likely
Damaged, chafed or shorted-to-earth signal wire in the sensor circuitlikely
Corroded, loose or contaminated connector at the sensor or control unitlikely
Open circuit or high resistance in the sensor reference supply or earthpossible
Carbon build-up in the EGR valve or passages affecting the sensor or its readingspossible
Faulty EGR valve assembly with integrated sensorpossible
Fault within the engine control unitrare

How to diagnose

Read all stored codes and freeze frame data to see engine load, temperature and speed when the fault set, and note any related EGR codes.
Check the manufacturer's information to identify which sensor is 'B' on this model and engine, and how it is wired.
Visually inspect the sensor, its connector and the wiring for heat damage, chafing, corrosion, water ingress or loose pins.
Clear the code and check whether it returns straight away or only under certain conditions, which helps separate a fixed fault from an intermittent one.
With a scan tool, view live sensor data and compare it with the expected values; a reading stuck low suggests a sensor or wiring fault.
Using a multimeter and the wiring diagram, check the sensor reference voltage, earth and signal wire for continuity and shorts to earth.
Inspect the EGR valve and passages for heavy carbon build-up or sticking, following the manufacturer's procedure.
If wiring and supply are good, test or substitute the sensor or valve assembly as the manufacturer advises before suspecting the control unit.

How to fix

Replace the faulty EGR sensor or the EGR valve assembly if the sensor is not sold separately.
Repair or replace damaged wiring and fix any short to earth or open circuit.
Clean or replace corroded connectors and terminals and make sure they are fully seated.
Carefully clean carbon deposits from the EGR valve and passages where the manufacturer permits this, or replace the parts if worn.
Repair or replace the control unit only after all wiring 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 a repair soon as the EGR system may not work properly, which can affect emissions, running quality and engine wear. If the engine runs very roughly, stalls or goes into limp mode, limit driving until it is checked.

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