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, DTC P0401 will set. 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 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. |
| • | BARO is more than 70 kPa. |
| • | Engine coolant temperature is more than 75°C (167°F). |
| • | Intake air temperature is more than 80°C (176°F). |
| • | Engine speed is between 1000 RPM and 1400 RPM. |
| • | IAC position is steady. |
| • | Vehicle speed is more 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 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 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). |
| • | During the deceleration condition, if the engine speed increases
more than 100 RPM from the enabling RPM (1000-4000 RPM), the
PCM will reset the test. |
| • | During the deceleration condition, if the engine speed decreases
more than 300 RPM from the enabling RPM (1000-4000 RPM), 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 the following conditions: |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wire connection |
| • | 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 malfunction. |
If the DTC cannot be duplicated and is determined to be
intermittent, reviewing the Failure Records can be useful in determining
when the DTC was last set. Also refer to
Testing for Intermittent Conditions and Poor Connections
in Wiring Systems.
DTC P0401 - EGR Flow Insufficient Detected
Step
| Action
| Values
| Yes
| No
|
1
| Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?
| --
|
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 as necessary.
Did you find and correct the condition?
| --
|
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.
Did you find and correct the condition?
| --
|
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.
Did you find and correct the condition?
| --
|
Go to Step 5
| Go to Diagnostic Aids
|
5
|
- Review and record the scan tool Failure 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 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
|
