What it means
Exhaust Gas Recirculation Sensor "B" Circuit Low: the signal voltage is below the expected range. Without valid EGR feedback the ECU disables exhaust gas recirculation and may set further EGR codes.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The EGR system recirculates exhaust gas to help control combustion temperature and emissions. The control module monitors EGR sensor B's feedback circuit and sets P0408 when the signal is higher than the expected range.
Common causes
Most likely first.
- EGR position sensor B failed
- Wiring or connector fault
- Connector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Rough idle or hesitation
- Reduced engine performance
- Increased emissions
How to diagnose it
- Record freeze-frame data and check live EGR sensor B readings to confirm when the high signal occurs and whether it changes with commanded EGR operation.
- Inspect sensor B's connector and harness for corrosion, loose terminals, damaged insulation, or contact with a power source.
- Check the connector terminals for poor contact and verify the sensor's supply and ground circuits using the vehicle wiring diagram.
- Test the sensor B signal wire for a short to voltage and check circuit continuity with the circuit safely isolated as specified by the manufacturer.
- Compare sensor B output and circuit tests with the manufacturer's specification to identify a sensor or wiring fault.
Common fixes
- Clean and secure corroded or loose sensor B connector terminals.
- Repair wiring shorted to a power source or any damaged circuit section.
- Replace EGR position sensor B if it fails the manufacturer's specification.
Can I keep driving?
A brief drive may be reasonable if the engine runs normally, but avoid hard acceleration and towing if it hesitates or loses power. The fault can impair EGR control and raise emissions; stop if rough running or power loss makes driving unsafe.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.