Technical Definition:

Exhaust Gas Recirculation (EGR) (Sensor B) Circuit High

Trouble Code: P0408
Address: 0x0408
System: Powertrain
Subsystem: Auxiliary Emission Controls
Technical Description:

The EGR (Exhaust gas recirculation) system is designed to lower combustion temperatures under certain conditions, thereby reducing oxides of Nitrogen (NOx) emissions. It does this by recirculating inert exhaust gasses back into the cylinder to be burned with the air/fuel mixture. Inert exhaust gas burns slower and lowers the temperatures of combustion. The EGR valve is computer controlled and opens to allow the exhaust gas to flow into the cylinders.

Possible Causes:
  • EGR sensor signal circuit shorted to B+ (battery voltage)
  • EGR sensor signal circuit shorted to the 5 volt reference circuit to EGR
  • EGR sensor ground circuit open
  • EGR sensor signal circuit open
  • Bad EGR (internal failure on EGR sensor or solenoid)
  • Debris caught in valve and holding it open or closed

Possible Symptoms:
  • Increase in combustion temperatures (and NOx emissions)
  • MIL (Malfunction indicator lamp) illumination
  • Possible surging while driving
  • Possible intermittent stall

Possible Solutions:

OBD-II Trouble Code Type:

Diagnostic Step:

Tech Notes:

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