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 sense a proportional
increase in MAP. If the MAP does not increase , 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, the PCM will set DTC P0401. 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 12
test samples during the first ignition cycle following a scan tool Clear
Info or a battery disconnect. Between 9 and 12 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
| • | None of the following DTCs are set: |
| - | Throttle position (TP) sensor |
| - | Vehicle speed sensor (VSS) |
| - | Intake air temperature (IAT) sensor |
| - | Manifold absolute pressure (MAP) sensor |
| - | Engine coolant temperature (ECT) sensor |
| - | Crankshaft position (CKP) sensor |
| - | Mass air flow (MAF) sensor |
| • | The BARO is more than 70 kPa. |
| • | The engine coolant temperature is more than 75°C (167°F). |
| • | The engine speed is between 1000 RPM and 1400 RPM
(VIN K). |
| • | The engine speed is between 900 RPM and 1300 RPM
(VIN 1). |
| • | The IAC position is steady. |
| • | The vehicle speed is more than 25 MPH. |
| • | The throttle is closed. |
| • | The commanded state of the A/C relay is steady. |
| • | The transaxle gear and TCC state are steady. |
Conditions for Setting the DTC
The 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 malfunction indicator lamp (MIL) during
the third consecutive trip in which the diagnostic has 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, check the exhaust system
for blockage, possibly a plugged catalytic converter, using the Restricted
Exhaust System Check.
To verify a repair, clear info with the 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 flow test during a closed throttle
condition. |
| • | The PCM will only run the EGR flow test at vehicle speeds of more
than 45 km/h (28 mph). |
| • | 9-12 deceleration cycles will be necessary to run a sufficient
number of EGR flow test samples. |
| • | 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. |
Check for the following conditions:
| • | A 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. Check for objects blocking
the vacuum to the MAP sensor. Also check for kinked or pinched
MAP sensor vacuum hoses (if equipped.) |
| • | A faulty connection or a damaged harness--Inspect the wiring
harness for damage. Use a corresponding mating terminal to test for proper
terminal tension. |
| • | If the harness appears to be OK, observe the Actual EGR Position
display on the scan tool while moving the connectors and the wiring harnesses
related to the EGR valve. A change in the display will indicate
the location of the malfunction. |
Reviewing the Failure Records vehicle mileage since the diagnostic test
last failed may help determine how often the condition that caused the DTC
to be set occurs. This may assist in diagnosing the condition.
DTC P0401 - EGR Flow Insufficient Detected
Step
| Action
| Values
| Yes
| No
|
1
| Was the Powertrain On Board Diagnostic (OBD) System Check performed?
| --
|
Go to Step
2
| Go to
Powertrain On Board Diagnostic (OBD) System Check
|
2
|
- Inspect the exhaust system for modification of the original installed
parts or leaks.
- If a problem was found, repair the exhaust system as necessary.
Was a problem found?
| --
|
Go to Step 5
|
Go to Step 3
|
3
|
- Remove the EGR valve. Refer to
Exhaust Gas Recirculation Valve Replacement
.
- Visually and physically inspect the following items:
| • | The pintle, the valve passages, and the adapter for excessive
deposits or any kind of a restriction |
| • | The EGR valve gasket and pipes for leaks |
- If a problem is found, clean or replace the EGR system components
as necessary.
Was a problem found?
| --
|
Go to Step 5
|
Go to Step 4
|
4
|
- Remove the EGR inlet and outlet pipes from the exhaust manifold
and the intake manifold.
- Inspect the manifold EGR ports and the EGR inlet and outlet pipes
for a blockage caused by excessive deposits or other damage.
- If a problem is found, correct the condition as necessary.
Was a problem found?
| --
|
Go to Step 5
| Go to Diagnostic Aids
|
5
|
- Review and record the scan tool Failure Records data.
- Clear the DTC and monitor the EGR Test Count display on the scan
tool while operating the vehicle as specified in the DTC P0401 Diagnostic
Aids.
- Continue operating the vehicle until 9-12 test samples have
been taken.
- Select the scan tool Specific DTC information for DTC P0401.
- Note the test result.
Does scan tool indicate the DTC P0401 test Ran and Passed?
| --
| System OK
|
Go to Step
2
|
