P0408 code meaning
EGR Sensor B Circuit High
Powertrain system Generic code P04xx codes Check engine light
What P0408 means
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.
P0408 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.5 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0408
- Check engine light on
- Usually no change in driving
- EGR or EGR cooler protection possibly restricted
Common causes of P0408
- Most likelySignal or ground wire open, connector off
- PossibleSensor faulty
- PossibleConnector corroded (near coolant or exhaust)
- Less likelyEngine module input
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
EGR sensor bEGR cooler coolant temperature sensorSensor wiringEngine control module
What each part of P0408 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 08 | The exact fault in that area. |
What to do next
- Note P0408 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0408
What does P0408 mean?
P0408 means "EGR Sensor B Circuit High". Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx. It is a generic code, so it means the same on every make.
Is it safe to drive with P0408?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0408 fail an emissions test?
Yes, it can. P0408 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0408 to fix?
Difficulty: easy. A typical repair takes 0.5 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0408 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.