A short to voltage on the sensor 'B' signal wire, often from chafed insulation against a hot or moving component; An open in the sensor 'B' ground circuit, causing the signal voltage to float high; A failed EGR position sensor 'B' that is internally shorted or stuck at the high end of its travel; A stuck or carbon-fouled EGR valve that cannot close fully, holding the sensor at a high position; Corroded, loose, or pushed-back terminals at the EGR sensor or engine computer connector; A faulty engine computer driver or internal pull-up resistor, though this is rare; Aftermarket electrical accessories improperly tapped into the sensor reference or signal circuit

DTC codes / P04xx · Auxiliary emission controls / P0408

P0408Exhaust Gas Recirculation Sensor 'B' Circuit High

Q: What does code P0408 mean?

Exhaust Gas Recirculation Sensor 'B' Circuit High. P0408 means the engine computer has detected an abnormally high voltage on the Exhaust Gas Recirculation (EGR) sensor 'B' circuit, which is typically the sensor that reports the actual EGR valve position. The computer expects a voltage within a calibrated range as the valve moves, and a sustained high reading tells it the circuit is shorted to power, open, or the sensor is stuck at the top of its range.

P0408 means the engine computer has detected an abnormally high voltage on the Exhaust Gas Recirculation (EGR) sensor 'B' circuit, which is typically the sensor that reports the actual EGR valve position. The computer expects a voltage within a calibrated range as the valve moves, and a sustained high reading tells it the circuit is shorted to power, open, or the sensor is stuck at the top of its range.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaAuxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Rough idle or stalling when coming to a stop, because the EGR valve may be stuck open or the computer commands incorrect EGR flow
Pinging or knocking under light acceleration due to excessive EGR flow or incorrect timing
Failed emissions inspection because the EGR monitor will not complete or the fault is active
Slightly reduced fuel economy or poor driveability if the EGR system remains disabled
In some vehicles, a limp-home mode with reduced power or a message on the driver information display

Common causes, most likely first

A short to voltage on the sensor 'B' signal wire, often from chafed insulation against a hot or moving component
An open in the sensor 'B' ground circuit, causing the signal voltage to float high
A failed EGR position sensor 'B' that is internally shorted or stuck at the high end of its travel
A stuck or carbon-fouled EGR valve that cannot close fully, holding the sensor at a high position
Corroded, loose, or pushed-back terminals at the EGR sensor or engine computer connector
A faulty engine computer driver or internal pull-up resistor, though this is rare
Aftermarket electrical accessories improperly tapped into the sensor reference or signal circuit

How to diagnose and fix it

Scan all modules and record freeze-frame data for P0408, then inspect the EGR sensor 'B' connector and wiring for obvious damage, corrosion, or loose pins
With the key on and engine off, back-probe the sensor 'B' signal wire and compare voltage to the manufacturer's specification; a reading near 5 volts with the valve commanded closed points to a short or open ground
Disconnect the sensor and check for a short to voltage on the signal wire using a multimeter, then check the ground circuit for excessive resistance
Command the EGR valve through its full range with a scan tool while watching sensor 'B' voltage; a voltage that does not change smoothly indicates a stuck valve or faulty sensor
Remove and inspect the EGR valve for carbon buildup or mechanical binding, cleaning or replacing it as needed, and verify the sensor moves freely
If wiring and valve are good, substitute a known-good sensor 'B' and clear the code to see if it returns
After repairs, clear codes and complete the EGR monitor drive cycle to confirm the fix before returning the vehicle

What a high EGR sensor reading really means

A high voltage on sensor 'B' does not always mean the sensor itself has failed; an open ground circuit will cause the signal wire to float high and set the same code, so always check the ground before replacing parts.
Many technicians replace the EGR valve only to find the code returns because the connector terminals were spread or corroded, creating intermittent high resistance that mimics a sensor fault.
On some vehicles, sensor 'B' is the second EGR position sensor used for rationality checks, and a high reading can be caused by a misadjusted or carbon-stuck valve that never reaches the closed position the computer expects.
A short to voltage on the sensor signal wire can be caused by aftermarket wiring routed too close to the EGR pipe or by a previous repair that pinched the harness, so inspect for non-factory splices.
The engine computer may set P0408 only after two consecutive trips with the fault present, so a single high reading from a loose connector may not store the code until the fault repeats.

Can I keep driving with P0408?

You can usually drive the vehicle with P0408, but the EGR system may be disabled and emissions will be affected; avoid heavy acceleration or long uphill pulls if you notice pinging, and have the fault diagnosed soon to prevent carbon buildup or catalytic converter damage from incorrect EGR flow.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.