P0401

DTC P0401 EGR System Flow Insufficient




Circuit Description
An Exhaust Gas Recirculation (EGR) system is used to lower Oxides of Nitrogen (NOx) emission levels caused by high combustion temperatures. It accomplishes this by feeding small amounts of exhaust gases back into the combustion chamber. When the air/fuel mixture is diluted with the exhaust gases, combustion temperatures are reduced.

A linear EGR valve is used on this system. The linear EGR valve is designed to accurately supply exhaust gases to the engine without the use of intake manifold vacuum. The valve controls exhaust flow going into the intake manifold from the exhaust manifold through an orifice with a VCM controlled pintle valve. The VCM commands EGR by applying a 12 volt duty cycle to the EGR valve feed control circuit.

This can be monitored on a scan tool as Desired EGR position.

The VCM can monitor the pintle position with the EGR pintle position signal. This sensor uses a variable resistor with a 5 volt reference, a signal circuit and a ground. The signal circuit monitors changes in voltage on the variable resistor as the pintle moves.

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 MAP sensor will detect an increase in manifold pressure as the EGR is cycled ON. If the amount of manifold pressure observed is less than expected by the VCM, a DTC P0401 will be set.

Conditions for Running the DTC
- No active TP DTCs
- No active MAP DTCs
- No active VS DTCs
- No active IAT DTCs
- No active ECT DTCs
- No active IAC DTCs
- No active misfire DTCs
- No active EGR pintle position DTCs
- The ECT is greater than 70 °C (158 °F)
- The BARO is greater than 70 kPa
- The vehicle speed is greater than 27 mph (43 km/h)
- The change in IAC is less than 8 counts
- The AC clutch status is unchanged
- The transmission status is unchanged
- The throttle position is less than 1 percent.
- The EGR position is less than 1 percent.
- The engine speed is between 1,000 - 1,900 RPM (manual transmission).
- The engine speed is between 800 - 1,600 RPM (automatic transmission).
- Any change in MAP is less than 0.5 kPa.
- DFCO not active (automatic transmission)
- No change in DFCO status (manual transmission)

Conditions for Setting the DTC
The change in MAP is less than a calibrated value for more than 2 seconds.

Action Taken When the DTC Sets
The VCM illuminates the MIL when this DTC is set.

Conditions for Clearing the MIL/DTC
- The VCM turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
- 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 is more than 70 °C (158 °F) during the same ignition cycle).
- Use the scan tool Clear Information function.

Diagnostic Aids

NOTE: 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

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

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.

An intermittent may be caused by any of the following conditions:
- A poor connection
- Rubbed through wire insulation
- A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis.

If a repair is necessary, then refer to Wiring Repairs or Connector Repairs.

Step 1:

Steps 2 - 4:

Steps 5 - 11:

Steps 12 - 16:

Step 17:

Test Description
The numbers below refer to the step numbers in the diagnostic table:
2. This test ensures that the EGR valve is allowing EGR flow and the VCM is capable of controlling the EGR valve.
3. This step checks the MAP sensors ability to correctly indicate barometric pressure.

In order to determine the correct MAP sensor display of barometric pressure for any elevation perform either of the following:
- Compare the recorded MAP sensor display value to the BARO display on a known good vehicle.
- Obtain the local weather service barometer reading (in Hg). Multiply the barometer reading by 3.4. The result indicates the correct BARO value for your area.

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.