The linear EGR system consists of a linear EGR valve, the PCM, and its associated wiring. The PCM directly controls the ground for the EGR valve when certain conditions have been met. Voltage is supplied by the UHJB to the EGR valve when the ignition is turned ON. When the PCM grounds the valve, a diagnostic feedback signal can determine if the valve had actually moved. Since the correct feedback voltage is possible, but the passage has restricted flow, DTC P0401 was implemented.
EGR flow restrictions are diagnosed during a deceleration. The EGR valve is commanded closed during a normal deceleration. To run the DTC P0401 test, the valve is opened during deceleration and a change in MAP is expected. If the change in MAP is below a threshold, it indicates flow is restricted.
The EGR flow insufficient test is checked
once per ignition cycle during deceleration.
The EGR flow insufficient test is checked once per ignition cycle during deceleration.
DTC P0401 will set if a calculated value compared to an engine speed vs barometric pressure determines there is a restriction in EGR flow when:
The scan tool can read the number of deceleration tests that have been performed since the DTCs were cleared. The scan tool also displays the EGR decel average value as well as the FAIL threshold when the DTC will set. The average must be greater than the threshold to set DTC P0401.