P0401

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

The VCM tests the EGR System during deceleration by momentarily commanding the EGR valve open. The pressure in the intake manifold increases when the EGR valve is open. The VCM monitors the Manifold Absolute Pressure (MAP) sensor signal during the EGR system diagnostic in order to determine the amount of EGR flow. The VCM calculates an error based on the difference between the actual MAP increase and a predetermined value. The VCM records the error and adjusts an internal fail counter towards the fail threshold if the error is too great. The VCM sets a DTC P0401 when the fail counter exceeds the fail threshold. The number of test samples required to exceed the fail threshold varies according to the magnitude of the detected flow errors.

The VCM allows only one EGR flow test sample to be taken during an ignition cycle. Following a scan tool Clear Info or a battery disconnect, the VCM allows twelve test samples during the first ignition cycle in order to aid in verifying a repair.

CONDITIONS FOR SETTING THE DTC

The following conditions will set the DTC:

^ No TP sensors DTCs

^ No MAP sensors DTCs

^ No VS sensors DTCs

^ No IAT sensors DTCs

^ No ECT sensors DTCs

^ No IAC sensors DTCs

^ No EGR Pintle position DTCs

^ No misfire DTCs

^ No EVAP DTCs

^ ECT is greater than or equal to 69.8°C

^ BARO is greater than or equal to 70 kPa

^ Vehicle speed is greater than 27 mph

^ Change in IAC is less than 6 counts on an automatic transmission

^ Change in IAC is less than 6 counts on an manual transmission

^ Transmission status is unchanged IAC clutch status is unchanged

Start Test

^ Throttle position is less than 2%

^ EGR position is less than 1%

^ Engine speed is greater than 1000 rpm but less than 1900 rpm (manual transmission)

^ Engine speed is greater than 800 rpm but less than 1600 rpm for the (automatic transmission)

^ Change in MAP is less than 1 A/D count (0.39 kPa)

^ Compensated MAP is greater than 20 kPa but less than 38 kPa

^ Not in DFCO (automatic transmission)

^ No change in DFCO state (manual transmission)

Run Test

^ Stabilized MAP (valve closed) recorded

^ EGR valve ramped open

^ Peak MAP value recorded

^ Change in MAP Computed

^ EGR valve closed

Rapid Step Response Enable criteria

^ MAP difference step is greater than 3.5 kPa

^ MAP difference is greater than -0.3 kPa

ACTION TAKEN WHEN THE DTC SETS

With a current DTC set, the VCM turns ON the MIL.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The VCM turns the MIL OFF after 3 consecutive drive trips when the test has Run and Passed and not Failed.

^ A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle).

^ Use the scan tool Clear Information function.

^ Disconnect the VCM battery feed for 30 seconds.

DIAGNOSTIC AIDS

NOTICE: In order to prevent further damage 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 Control Module.

Check the oil for possible fuel contamination if a stuck open fuel injector is found.

Check for the following conditions:

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

- Backed out terminal EGR Control circuit

- 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 EGR Control circuit 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 Valve Feed Control circuit is not shorted to voltage.

4. Ensures that the EGR circuits are not open and that the VCM is functioning properly.

16. Clearing the DTCs is a very important step in this diagnostic procedure. The clearing function allows the EGR Valve to relearn a new minimum pintle position, as the old position was inaccurate due to the malfunction that caused this DTC.