Circuit Description
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.
Conditions for Setting the DTC
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.
Important: 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:
| • | Engine speed is between 1,350-2,000 RPM. |
| • | The VSS is greater than 48 km/h (30 mph). |
| • | The throttle is closed. |
| • | The engine coolant temperature is greater than 70°C (150°F). |
| • | The vehicle is in top gear. |
| • | No EGR pintle, TP sensor, MAP, VSS, IAT, ECT, or IAC DTCs have been set. |
Diagnostic Aids
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.
| • | Carbon build-up around the EGR pintle valve bore opening may restrict
exhaust gas flow. Build-up of carbon in the EGR passages will result in DTC P0401. |
| • | Diagnose the other DTCs listed under parameters, especially a MAP sensor
DTC, before diagnosing DTC P0401. |
