Circuit Description
The PCM tests the 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,
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 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 Setting the DTC
| • | No TP sensor, Vehicle Speed Sensor, Misfire, IAT sensor, MAP sensor,
Idle speed, EGR Pintle Position sensor, ECT sensor, or MAF sensor DTC(s) set. |
| • | BARO is greater than 70 kPa. |
| • | Engine coolant temperature is greater than 80°C (180°F). |
| • | Engine speed is between 900 RPM and 1600 RPM. |
| • | IAC position is steady. |
| • | Vehicle speed is greater than 29 km/h (18 MPH). |
| • | The throttle angle is less than 1%. |
| • | MAP changes monitored during the EGR flow test indicate insufficient
EGR flow. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the 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 Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF 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 the scan tool Clear Information
function or by disconnecting the PCM battery feed. |
Diagnostic Aids
Important: If the EGR valve shows signs of excessive heat, check the exhaust system
for blockage (possibly a plugged catalytic converter). Refer to
Restricted Exhaust System Check
.
To verify a repair, clear the information 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 test during a closed throttle condition. |
| • | The PCM will only run the EGR test at vehicle speeds greater than
29 km/h (18 mph). |
| • | Several deceleration cycles will be necessary to run a sufficient
number of EGR flow test samples (9-12). |
| • | 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:
| • | 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.) |
| • | Poor connection at the PCM - Inspect harness connectors for the
following conditions: |
| - | Improperly formed or damaged terminals. |
| - | Poor terminal to wire connection. |
| • | 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 will indicate the location of the
fault. |
Review the Fail Records vehicle mileage since the diagnostic test last
failed. This may help determine how often the condition that caused the DTC
to be set occurs.
Step
| Action
| Value(s)
| Yes
| No
|
1
| Was
the Powertrain On-Board Diagnostic (OBD) System Check performed?
| --
|
Go to Step 2
| Go to
the
Powertrain On Board Diagnostic (OBD) System Check
|
2
|
- Inspect the exhaust system for modification of original installed
parts or leaks.
- If a problem was found, repair 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 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. Refer to EGR Pipes
in 3800
Engine On-Vehicle Service.
- 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
| Refer to Diagnostic Aids
|
5
|
- Review and record the scan tool Fail Records data.
- Clear DTC and monitor EGR Test Count display on the scan tool
while operating the vehicle as specified in DTC P0401 Diagnostic Aids.
- Continue operating the vehicle until 9 -12 EGR Test Count test
samples have been taken.
- Select scan tool Specific DTC information for DTC P0401.
Does scan tool indicate DTC P0401 Test Ran and Passed?
| --
| System OK
|
Go to Step 2
|
