GM/Detroit 6.5L ENGINE CONTROLS – DTC P0400 Exhaust Gas Recirculation (EGR) System Performance

Refer to Emission Hose Routing Diagram

Circuit Description
The PCM operates a PWM solenoid to control the EGR valve. This solenoid is normally open. By providing a ground path the PCM energizes the solenoid which then allows vacuum to pass to the EGR valve. During normal operation, the PCM compares its desired MAF signal with the measured MAF signal and makes corrections in the duty cycle accordingly.

Conditions for Running the DTC
• The PCM performs this DTC diagnostic continuously.
• Baro greater than 75 kPa.
• The DTCs P0405, P0406, P0102 and P0103 are not set.
• MAF value is greater than or equal to 0.1484 g/cyl.

Conditions for Setting the DTC
• Lowest achieved EGR pressure at full EGR is less than look up table value (internal to PCM)
• All diagnostic set conditions met for 2 seconds.

Action Taken When the DTC Sets
• The PCM will shut down the EGR.
• The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive drive trip that the diagnostic runs and fails.
• The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the Failure Records will store this information. If the diagnostic reports a failure on the second consecutive drive trip, the Freeze Frame records the operating conditions at the time of failure and updates the Failure Records.

Conditions for Clearing the MiL/DTC
• The PCM will turn the MIL off after three consecutive trips without a fault condition.
• A History DTC cl ears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
The most likely cause of failure is a restricted vacuum line from the EGR valve to the vacuum tee (including the vacuum tee).
To run the diagnostic test the engine must be at the operating temperature, vehicle in drive at idle for approximately 1 minute. Then, with the vehicle in park, hold the engine rpm steady between 1500 and 2100 rpm for 30 seconds. If the diagnostic test fails to run, the vehicle must be driven.

The Adaptive Learn Matrix (ALM) is used to adjust the EGR vacuum control based on mass air flow (MAF). The ALM may change as a result of back pressure increases over the life of the vehicle or other engine system variations. The ALM is made up of sixteen cells (numbered from zero to fifteen) in which each cell covers a range of engine speed (RPM) and load (mm3).

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. This step checks for the correct amount of vacuum at the EGR valve.
3. At zero vacuum and at 15 in. Hg, there should be a difference greater than 0.10 g/cyl. If the difference is not greater than 0.10 g/cyl the EGR valve is malfunctioning.


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