Circuit Description
The powertrain control module (PCM) tests the exhaust gas recirculation (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 PCM monitors the manifold absolute pressure (MAP) sensor signal during the EGR system diagnostic in order to determine the amount of EGR flow. The PCM calculates an error based on the difference between the actual MAP increase and a predetermined value. The PCM records the error and adjusts an internal fail counter towards the fail threshold if the error is too great. The PCM 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 PCM 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 PCM allows twelve test samples during the first ignition cycle in order to aid in verifying a repair.
Conditions for Running the DTC
| • | No active MAP DTCs |
| • | No active engine coolant temperature (ECT) DTCs |
| • | No active intake air temperature (IAT) DTCs |
| • | No active throttle position(TP) DTCs |
| • | No other active EGR DTCs |
| • | No active vehicle speed sensor (VSS) DTCs |
| • | No active idle air control (IAC) DTCs |
| • | No active system voltage DTCs |
| • | The ignition voltage is between 11.7 volts and 18 volts. |
| • | The BARO is more than 70 kPa. |
| • | The ECT is between 60°C (140°F) and 117°C (243°F). |
| • | The IAT is less than 65°C (140°F). |
| • | The engine speed is between 800 RPM and 2,000 RPM. |
| • | The TP sensor angle is less than 1.0 percent. |
| • | The engine vacuum is between 60 kPa (9 psi) and 83 kPa (12 psi). |
| • | The IAC position is steady. |
| • | The vehicle speed is between 42 km/h (26 mph) and 113 km/h (70 mph). |
| • | The change in VSS is less 6 km/h (4 mph). |
| • | The change in MAP is less than 0.5 kPa. |
| • | The A/C clutch status is steady. |
| • | The fuel system is not in deceleration fuel cut-off (4.8L only). |
Conditions for Setting the DTC
| • | The change in the MAP is less than a predetermined value during the EGR flow test. |
| • | All conditions are present for 2 seconds. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The PCM stores this information in the Freeze Frame and/or Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A last test failed, or current DTC, clears when the diagnostic runs and does not fail. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Use a scan tool in order to clear the MIL and the DTC. |
Diagnostic Aids
Important: Remove any debris from the PCM connector surfaces before servicing the PCM. Inspect the PCM connector gaskets when diagnosing or replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets prevent water intrusion into the PCM.
Use a scan tool in order to clear information. Run the EGR flow test in order to verify a repair. Keep the following items in mind when running the EGR flow test:
| • | The PCM runs the EGR flow test during a gradual deceleration. |
| • | The PCM runs the EGR flow test during a closed throttle condition. |
| • | The PCM runs the EGR flow test at vehicle speeds between 42 km/h (26 mph) and 113 km/h (70 mph). |
| • | Several deceleration cycles will be necessary to run a sufficient number of EGR flow test samples (9-12). |
Inspect for the following conditions:
| • | 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. |
| • | An engine that is running poorly may cause this DTC to set. |
| • | Poor connection or damaged harness -- Inspect the wiring harness for damage. |
| • | If the harness appears to be OK, observe the Actual EGR Position display on the scan tool while moving connectors and wiring harnesses related to the EGR valve. A change in the display indicates the location of the malfunction. |
Reviewing the Failure Records vehicle mileage since the diagnostic test last failed helps determine when the condition that caused the DTC to be set occurred. This may assist in diagnosing the condition.
Test Description
The number below refers to the step number on the diagnostic table.
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This DTC is set when the PCM does not detect a predetermined change in MAP when the EGR valve is commanded. This step checks for any MAP sensor DTCs. A malfunctioning MAP sensor may cause this DTC to set.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||
Is there any MAP DTCs set? | -- | Go to the applicable DTC table | ||||||
3 |
Did you find and correct the condition? | -- | ||||||
4 |
Did you find and correct the condition? | -- | ||||||
5 |
Did you find and correct the condition? | -- | ||||||
6 |
Did you find and correct the condition? | -- | Go to Diagnostic Aids | |||||
7 |
Does the scan tool indicate that this test failed? | -- | ||||||
8 | Select the Capture Info option and the Review Info option using the scan tool. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to the applicable DTC table | System OK |
