P0401

Code P0401, Exhaust Gas Recirculation (EGR) Flow Test Failure:

Exhaust Gas Recirculation (EGR) Circuit:

CIRCUIT DESCRIPTION
The PCM tests the exhaust gas recirculation valve by momentarily commanding the valve "ON" while monitoring engine RPM, If the expected drop is not seen for a calibrated number of tests, the PCM will illuminate the MIL (Service Engine Soon) and store DTC P0401.

The PCM will run the EGR tests when the following conditions are met:
^ Engine coolant temperature is over 85°C (185°F).
^ Throttle angle below 1%.
^ Brake has been applied.
^ Vehicle speed above 25 mph.
^ Engine speed between 900 and 1100 RPM.
^ DTC P0122, P0123, or P0501 not set.

DTC P0401 WILL SET WHEN:
^ The EGR flow test will be failed and DTC P0401 set if the PCM does not detect a drop in engine speed of at least 50 RPM for 6 out of 8 tests. Once 3 tests are run during an ignition cycle, the PCM will not run the flow test until the next ignition cycle. Therefore, it will take 3 or more ignition cycles to set DTC P0401.

ACTION TAKEN (PCM will default to):
With a current DTC P0401 set, the PCM will illuminate the MIL (Service Engine Soon).

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Ensures that the EGR valve is allowing EGR flow and the PCM is capable of controlling the EGR valve.
2. Ensures that CKT 435 is not shorted to ground.
3. Ensures that CKT 435 is not open and that the PCM quad-driver for CKT 435 is functioning properly.


DIAGNOSTIC AIDS
An intermittent may be caused by a wire broken inside the insulation

Check for:
^ Poor connection or damaged harness. Inspect PCM harness connectors for backed out terminal "RA16", improper mating, broken locks, improperly formed or damaged terminal, poor terminal to wire connection and damaged harness.
^ Intermittent test - If connections and harness check OK, monitor a digital voltmeter connected between terminal "RA16" and ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.

NOTE: If the EGR valve shows signs of excessive heat, check the exhaust system for blockage (possibly a plugged converter) using the procedure found in "DIAGNOSTIC CHARTS/CHART B-1, RESTRICTED EXHAUST SYSTEM CHECK". B Charts


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;
A. Stuck.
B. Grounded driver circuit.
C. Possible faulty PCM.

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