P0401

CIRCUIT DESCRIPTION
The VCM tests the exhaust gas recirculation valve by momentarily commanding the valve on while monitoring the MAP. If the expected MAP increase is not seen under certain operating conditions (decel) for a calibrated number of tests, the VCM turns on the MIL (Malfunction Indicator Lamp), and the VCM stores the DTC P0401. This is a type A DTC.

The VCM runs the EGR tests when the following conditions are met:

^ No TP sensor DTCs set.
^ No MAP DTCs set.
^ No VS sensor DTCs set.
^ No IAT sensor DTCs set.
^ No ECT sensor DTCs set.
^ No IAC DTCs set.
^ No linear EGR pintle position DTCs set.
^ No Transmission DTCs set.
^ The change in IAC is less than 8 counts.
^ ECT greater than 78°C.
^ Baro greater than 70 kPa.
^ Vehicle speed greater than 27 mph.
^ AC clutch status is unchanged.
^ Transmission is in locked or unlocked stage (stable).
^ No Misfire DTCs set.

CONDITIONS FOR SETTING THE DTC
The following conditions will set the DTC:

^ The EGR flow test fails if the VCM does not detect a sufficient rise in the MAP during the above conditions.
^ The DTC P0401 sets if the VCM does not detect a sufficient rise in the MAP during the above conditions.

ACTION TAKEN WHEN THE DTC SETS
With a current DTC set, the VCM turns on the Malfunction Indicator Lamp (MIL).

CONDITIONS FOR CLEARING THE MIL/DTC
The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

DIAGNOSTIC AIDS

NOTE: If the EGR valve shows signs of excessive heat, check the exhaust system for blockage (possibly a plugged converter) using the procedure found on the restricted exhaust system check. If the exhaust system is restricted, repair the cause; one of which might be an injector which is open due to one of the following reasons:

^ Stuck.
^ Grounded driver circuit.
^ Possible faulty VCM.

If this condition is found, the oil should be checked for possible fuel contamination.

Check for the following conditions:

^ Poor connection or damaged harness - Inspect VCM harness connectors for the following conditions:

^ Backed out terminal BL 18.
^ Improper mating.
^ Broken locks.
^ Improperly formed or damaged terminal.
^ Poor terminal to wire connection.
^ Damaged harness.

^ Intermittent test - If connections and harness check OK, monitor a digital voltmeter connected between terminal BL 18 and ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.

TEST DESCRIPTION
The numbers below refer to the step numbers in the diagnostic table.

2. Ensures that the EGR valve is allowing EGR flow and the VCM is capable of controlling the EGR valve,
3. Ensures that the EGR LOW signal circuit is not shorted to ground.
4. Ensures that the EGR LOW signal circuit is not open and that the VCM quad-driver for the EGR LOW signal circuit is functioning properly.