P0408EGR Sensor B Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx. What EGR sensor B is depends on the manufacturer – on systems with a second valve or pressure sensor it is that second sensor. GM's diesels of 2017/18 use P0408 for the coolant temperature sensor at the inlet of the high-pressure EGR cooler: high means a resistance above 134 kΩ, i.e. an open circuit or extreme cold (20 of 24 samples, MIL after two trips). A substitute value is used.
- What you may notice
- Check engine light on
- Usually no change in driving
- EGR or EGR cooler protection possibly restricted
- Likely causes
- LikelySignal or ground wire open, connector off
- PossibleSensor faulty
- PossibleConnector corroded (near coolant or exhaust)
- RareEngine module input
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €30 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.