Code P0408 indicates that the engine computer has detected an abnormally high voltage in the electrical circuit for the 'B' Exhaust Gas Recirculation (EGR) sensor. This typically points to a wiring issue, a short to power, or a failed sensor rather than a mechanical failure of the EGR valve itself.
The EGR sensor itself can fail internally, causing the signal circuit to short to the reference voltage inside the sensor.
The signal wire in the EGR sensor circuit may be chafed or melted, shorting against a power source and causing the high voltage reading.
Corroded, bent, or pushed-out pins in the sensor connector can alter the circuit resistance and cause a false high voltage condition.
If the ground wire for the EGR sensor is broken or disconnected, the sensor circuit can float up to the reference voltage, triggering a high input code.
The engine computer rarely fails in a way that causes this code, but an internal short on the circuit board is possible as a last-resort cause.
Not necessarily. It means the electrical circuit for the sensor has high voltage, which is often a wiring short or a failed sensor rather than a mechanical failure of the valve itself.
You can usually drive short distances, but the engine may run poorly or hesitate. Extended driving can lead to engine pinging or increased emissions, so it should be diagnosed promptly.
Replacing the PCM is almost never the fix for this code. High voltage codes are overwhelmingly caused by wiring shorts or failed EGR sensors; a PCM failure is extremely rare and should only be considered after all wiring and the sensor have been definitively proven good.
Some vehicles use multiple EGR sensors or position sensors to monitor the system. 'Sensor B' refers to the specific secondary sensor designated by the manufacturer, usually monitoring the position or flow of the EGR system.