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 thirteen
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 Running the DTC
| • | No TP sensor, VSS, Misfire, IAT sensor, MAP sensor, Idle speed,
EGR Pintle Position, ECT sensor, Fuel injector fault(s), CKP sensor, or MAF
sensor DTCs set. |
| • | System voltage is between 10 and 16 volts. |
| • | BARO is greater than 70 kPa. |
| • | Engine coolant temperature is greater than 75°C (167°F). |
| • | Intake air temperture is greater than 80°C (176°F). |
| • | Engine speed is between 1050 RPM and 1400 RPM. |
| • | IAC position is steady. |
| • | Vehicle speed is greater than 25 MPH. |
| • | The throttle is closed. |
| • | Commanded state of the A/C relay is steady. |
Conditions for Setting the DTC
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 Info 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) 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 test during a closed throttle condition. |
| • | The PCM will only run the EGR test at vehicle speeds above 25 MPH. |
| • | Several deceleration cycles will be necessary to run a sufficient
number of EGR flow test samples (9-12). |
| • | If the engine speed increases more than 250 RPM from the enabling
RPM (refer to Conditions for Running the DTC), the PCM will reset the test. |
| • | If the engine speed decreases more than 250 RPM from the enabling
RPM (refer to Conditions for Running the DTC), the PCM will reset the test. |
| • | When the PCM commands the EGR valve open engine speed will drop.
If the engine speed drops below 750 RPM, the test will not run. |
| • | 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 PCM or EGR Valve. |
| | Inspect harness connectors for backed out terminals, improper mating,
broken locks, improperly formed or damaged terminals, and poor terminal to
wire connection. |
| | 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 malfunction. |
Reviewing the Fail 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.
Step
| Action
| Value(s)
| 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 original installed
parts or leaks.
- If a problem was found, repair exhaust system as necessary.
Was a condition present that required repair?
| --
|
Go to Step 5
|
Go to Step
3
|
3
|
- Remove the EGR valve.
- Visually and physically inspect the following items:
| • | Pintle, valve passages and the adapter for excessive deposits
or any kind of a restriction. |
| • | EGR valve gasket and pipes for leaks. |
- If a problem is found, clean or replace EGR system components
as necessary. Refer to
Exhaust Gas Recirculation Valve Replacement
.
Was a condition present that required repair?
| --
|
Go to Step 5
|
Go to Step
4
|
4
|
- Remove the EGR exhaust 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, casting flashing or other damage.
- If a problem is found, correct the condition as necessary.
Was a condition present that required repair?
| --
|
Go to Step 5
| Go 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 Diagnostic Aids.
- Continue operating the vehicle EGR Test Count until 9 -
12 test samples have been taken.
- Select scan tool Specific DTC information for DTC P0401.
- Note test result.
Does scan tool indicate DTC P0401 Test Ran and Passed?
| --
| Repair complete
|
Go to Step 2
|
