Circuit Description
The engine control module (ECM) tests the flow of the exhaust gas recirculation (EGR) system during steady state driving conditions. The ECM does this by momentarily commanding the EGR valve to open while monitoring the signal of the manifold absolute pressure (MAP) sensor. The ECM performs one flow test per ignition cycle. When the EGR valve is opened, the ECM will expect to see a predicted increase in MAP. If the ECM detects a less than predicted increase in MAP, DTC P0401 sets.
Conditions for Running the DTC
| • | Accumulated air mass is more than 0.5 kg. |
| • | The EGR valve is commanded to 100 percent duty cycle. |
| • | The engine speed is between 1,600-2,200 RPM and is steady. |
| • | Engine load is between 35-65 percent. |
| • | Altitude is less than 2750 meters (9,023 feet). |
| • | The EGR flow test can take between 4-9 seconds to run. |
Conditions for Setting the DTC
| • | The MAP changes monitored by the ECM during the EGR flow test indicate an insufficient amount of EGR flow. |
| • | The above condition exists for more than 8 seconds. |
Action Taken When the DTC Sets
| • | The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
| • | Clear the MIL and the DTC with a scan tool. |
Diagnostic Aids
Inspect for the following conditions:
| • | A vacuum restriction to the MAP sensor--A skewed MAP sensor reading can cause the ECM to read incorrect MAP changes during the EGR flow test. |
| • | An engine that is running poorly due to a mechanical condition such as worn piston rings, worn camshaft, etc.--These types of conditions can cause low engine vacuum and can cause less than expected MAP changes during the flow test. |
| • | Excessive back pressure in the exhaust system may cause this DTC to set. This condition can cause low engine vacuum and thus can cause a less than expected MAP changes during the EGR flow test. Possible causes of this could be a restriction in the exhaust system or non original equipment manufacture (OEM) exhaust parts. |
| • | Exhaust system leaks can cause an insufficient amount of EGR flow through the EGR valve. This condition can cause a less than expected MAP changes due to insufficient exhaust back pressure. possible causes of this could be a leaking exhaust system, a leaking EGR pipe or non OEM exhaust parts. |
| • | A restriction in the intake manifold such as carbon deposits and casting flash |
Test Description
The number below refers to the step number on the diagnostic table.
-
MAP sensor, EGR control, and EGR position sensor faults must be diagnosed first. A skewed MAP sensor reading or EGR position sensor reading could cause this DTC to set.
Step | Action | Yes | No |
|---|---|---|---|
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |
Is DTC P0107, P0108, P0403, P0405, P0406, P0409, P0489, P0490, P1403, P1405, or P1406 also set? | Go to Step 3 | ||
3 | Inspect for a vacuum leak between the exhaust gas recirculation (EGR) valve and the intake manifold. Did you find and correct the condition? | Go to Step 9 | Go to Step 4 |
4 |
Did you find and correct the condition? | Go to Step 9 | Go to Step 5 |
5 | Inspect the EGR tube for leaks and restrictions. Did you find and correct the condition? | Go to Step 9 | Go to Step 6 |
6 | Inspect the manifold absolute pressure (MAP) sensor and gasket for vacuum leaks. Did you find and correct the condition? | Go to Step 9 | Go to Step 7 |
7 |
Did you find and correct the condition? | Go to Step 9 | Go to Step 8 |
8 | Inspect the exhaust system for leaks, restrictions, and for modification of original equipment manufacture (OEM) parts. Did you find and correct the condition? | Go to Step 9 | Go to Diagnostic Aids |
9 |
Did the DTC fail this ignition? | Go to Step 2 | Go to Step 10 |
10 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | System OK |
