Faulty EGR position or flow sensor 'B' reporting a signal that is too high; Open circuit in the sensor ground or reference return wire, pushing the signal high; Corroded, loose or contaminated connector terminals at the sensor or control unit; Signal wire shorted to the 5 volt reference or to battery voltage; Carbon build-up or a sticking EGR valve that stops the sensor reading correctly; Faulty EGR valve assembly where the sensor is built in; Fault inside the engine control unit.

P0408

EGR Sensor B Circuit High.

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
08: FaultThe specific fault in that area

What it means

P0408 is the generic code for a high input on the EGR sensor 'B' circuit. The engine control unit has seen the signal voltage from the second exhaust gas recirculation position or flow sensor sit above its expected range, which usually points to a short to voltage, an open circuit in the sensor ground, or a faulty sensor. Which sensor counts as 'B' and how it is arranged depends on the make, model and engine.

Symptoms

Engine warning light on
Rough or unstable idle
Hesitation or poor pickup under load
Slightly higher fuel consumption
Engine pinking or knocking under load
EGR system forced off, with possible limp-mode behaviour on some models

Possible causes

Faulty EGR position or flow sensor 'B' reporting a signal that is too highlikely
Open circuit in the sensor ground or reference return wire, pushing the signal highlikely
Corroded, loose or contaminated connector terminals at the sensor or control unitlikely
Signal wire shorted to the 5 volt reference or to battery voltagepossible
Carbon build-up or a sticking EGR valve that stops the sensor reading correctlypossible
Faulty EGR valve assembly where the sensor is built inpossible
Fault inside the engine control unitrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting engine speed, load, temperature and EGR commanded versus actual values.
Check for related codes such as other EGR, sensor reference voltage or throttle faults, and deal with those first.
Inspect the EGR sensor and valve wiring and connectors for damage, chafing, heat damage, corrosion, water ingress or loose pins.
With the ignition on, watch the live EGR sensor B reading and compare it against the specification for your model; the value should change smoothly as the valve is commanded.
Command the EGR valve open and closed with a scan tool, if supported, and check the sensor reading follows and the valve moves freely.
Check the sensor reference voltage and ground at the connector using a multimeter, following the wiring diagram for the vehicle.
Test the signal wire for a short to voltage and for continuity and resistance back to the control unit.
Remove the EGR valve and inspect it for carbon build-up or a stuck valve, following the model's procedure.

How to fix

Repair or replace damaged wiring, and clean or renew corroded connector terminals.
Restore the faulty ground or reference wire where an open circuit or short is found.
Replace the EGR sensor 'B' if it is a separate serviceable part and has failed testing.
Replace the EGR valve assembly if the sensor is integrated or the valve is mechanically faulty.
Clean carbon deposits from the EGR valve and passages where the manufacturer allows it, then refit with new gaskets.
Clear the codes after repair and road test to confirm the fault does not return.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but book it in soon because poor running and increased emissions may get worse. Avoid heavy loads and long trips if it runs badly or goes into limp mode.

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