P0401

Diagnostic Chart:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) tests the Exhaust Gas Recirculation (EGR) system during deceleration by momentarily commanding the EGR valve open while monitoring the Manifold Absolute Pressure (MAP) sensor signal. When the EGR valve is opened, the PCM should see a proportional increase in MAP. If the expected increase in MAP is not seen, the PCM notes the amount of error that was detected and adjusts an internal fail counter towards a fail threshold level. When the fail counter exceeds the fail threshold level, DTC P0401 will set. The number of test samples required to accomplish this may vary according to the amount of detected flow error.

Normally, the PCM will only allow one EGR flow test sample to be taken during an ignition cycle. To aid in verifying a repair, the PCM allows twelve test samples during the first ignition cycle following a scan tool Clear Info or a battery disconnect. Between nine and twelve samples should be sufficient for the PCM to determine adequate EGR flow and pass the EGR test if the system is operating correctly.

CONDITIONS FOR RUNNING THE DTC
^ No TP Sensor, VSS, Misfire, IAT Sensor, MAP Sensor, Idle Speed, EGR Pintle Position, ECT Sensor, Fuel Injector Faults, CKP Sensor, or MAF Sensor DTCs set.
^ BARO is more than 70 kPa.
^ Engine coolant temperature is more than 75°C (167°F).
^ Intake air temperature is more than 80°C (176°F).
^ Engine speed is between 900-1400 RPM.
^ IAC position is steady.
^ Vehicle speed is more than 25 mph.
^ The throttle is closed.
^ Commanded state of the A/C relay is steady.

CONDITIONS FOR SETTING THE DTC
MAP changes monitored during the EGR flow test indicate insufficient EGR flow.

ACTION TAKEN WHEN THE DTC SETS
^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the first trip in which the diagnostic test has been run and failed.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
^ The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

DIAGNOSTIC AIDS

IMPORTANT: If the EGR valve shows signs of excessive heat, inspect the exhaust system for blockage, possibly a plugged catalytic converter. Refer to Restricted Exhaust System Check. Component Tests and General Diagnostics

To verify a repair, clear info with a scan tool and run the EGR flow test keeping the following items in mind:
^ The PCM will only run the EGR flow test during a gradual deceleration.
^ The PCM will only run the EGR test during a Closed Throttle condition.
^ The PCM will only run the EGR test at vehicle speeds above 25 MPH.
^ Several deceleration cycles will be necessary to run a sufficient number of EGR flow test samples, 9-12.
^ If the engine speed increases more than 200 RPM from the enabling RPM the PCM will reset the test. Refer to Conditions for Running the DTC.
^ If the engine speed decreases more than 350 RPM from the enabling RPM the PCM will reset the test. Refer to Conditions for Running the DTC.
^ When the PCM commands the EGR valve open engine speed will drop. If the engine speed drops below 700 RPM, the test will not run.
^ The EGR Test Counter displayed on the scan tool can be useful in determining that the EGR flow test is running and to keep track of the number of test samples taken. The counter will increment each time a test sample is taken.

Inspect for vacuum restriction to the MAP sensor - A restriction to the MAP sensor can cause the EGR flow test to fail due to insufficient MAP changes being monitored during the test. Inspect for objects blocking the vacuum to the MAP sensor. Also, inspect for kinked or pinched MAP sensor vacuum hoses, if equipped.

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection-Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension.
^ Damaged harness-Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault.
^ PCM and engine grounds for clean and secure connections

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.