P0400

THIS ARTICLE HAS BEEN UPDATED BY TSB # 61-65-68, DATED JANUARY, 1997

Diagnostic Chart:

Wiring Diagram:

Circuit Description

The Exhaust Gas Recirculation (EGR) system controls the formation of Oxides of Nitrogen (NOx) emissions by recirculating the exhaust gases into the combustion chamber. The PCM uses expected changes in the Manifold Absolute Pressure (MAP) sensor readings to evaluate the performance of the EGR system. The PCM will apply full manifold vacuum to the EGR valve and then monitor the MAP sensor signal. The PCM judges the EGR flow to be faulty when the monitored MAP sensor pressure change is too large or too small.

Conditions for Running the DTC

^ Engine coolant temperature between 80°C (176°F) and 110°C (230°F).

^ Intake air temperature between -10°C (14°F) and 50°C (122°F).

^ Barometric pressure greater than or equal to 75 kPa.

^ Engine RPM between 1400 and 4000 RPM.

^ Fuel tank level greater than 25%.

^ Vehicle speed greater than 32 km/h (20 mph).

Conditions for Setting the DTC

Intake pressure change during steady state driving conditions:

^ Throttle position change is less than 0.244° in 16 firing events.

^ After 240 seconds from engine start in EGR control mode.

^ Intake pressure difference diagnostic will run for 2 seconds, once per drive cycle.

Intake pressure change during deceleration:

^ After 290 seconds from engine start in fuel shut-off mode.

^ Intake pressure difference diagnostic will run for 1 second, once per drive cycle.

Action Taken When the DTC Sets

^ The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.

^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC

^ The MIL will turn OFF after three consecutively passing cycles without a fault present.

^ A History DTC will clear after 40 consecutive warm-up cycles without a fault.

^ DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

Diagnostic Aids

Normal engine operating temperature is specified as 80°C (178°F) or above.

Check for any of the following conditions:

^ Vacuum hoses for poor connections, leakage, deterioration, clogged or incorrect routing.

^ EGR valve malfunction, stuck closed, stuck open, clogged.

^ EGR pressure transducer malfunction.

^ EGR solenoid vacuum valve and the EGR bypass valve and their circuitry for malfunctions.

^ The incorrect PCM can cause a DTC P0400 to set. Check that the correct PCM is installed in the vehicle as required for proper emission certification. California certified vehicles do not have an EGR system and therefore will set a DTC P0400 if a Federal or Canadian certified vehicle PCM is used.

^ A poor connection at the PCM. Inspect the harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal to wire connection.

^ A damaged wiring harness. Inspect the wiring harness for damage and repair as necessary.

For additional checks of the components listed above, Refer to Exhaust Gas Recirculation (EGR) system.

A DTC P0400 is set when the expected change in MAP is greater or less than the specified value, when the EGR valve is turned ON (opened) and then turned OFF (closed). Therefore any condition that can affect the accuracy of the MAP sensor readings should be investigated.

^ Retarded ignition timing.

^ Restricted exhaust system.

^ Faulty vacuum supply hose(s) to the MAP sensor.

If a DTC P0400 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

After all the checks under Exhaust Gas Recirculation (EGR) system have been completed and a malfunction continues to occur, replace the PCM with a known good unit and retest.

If any wiring repairs need to be made, Refer to Repair Procedures in Electrical Diagrams.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. Checks for an EGR valve that is staying open.

3. This step will determine if there are faults in the EGR valve or EGR By-Pass valve components. The EGR By-Pass valve output control is disabled by the PCM until the engine has reached a normal operating temperature of at least 80°C (176°F). Output controls can be found on the Tech 2 scan tool under Special Functions.

4. Determine if fault is present in the EGR pressure transducer or the EGR solenoid vacuum valve components. When the engine is cold the EGR valve should not operate when engine RPM is raised. Bring the engine to operating temperatures and raise the RPM. The EGR valve should move accordingly.

5. Driving the vehicle under these conditions will verify that the fault is present. If no DTC sets the fault is intermittent.

9. Determine the cause of the incorrect flow of exhaust gas.

14. After completing the diagnostic procedures and no open or short was found in the EGR By-Pass valve control circuit, replace the PCM with a known good unit and retest.

15. This step checks the operation of the EGR solenoid vacuum valve. The EGR solenoid vacuum valve output control is disabled by the PCM until the engine has reached a normal operating temperature of at least 80°C (176°F). Output controls can be found on the Tech 2 scan tool under Special Functions.

22. After the complete DTC table has been followed and no open or short was found in the EGR solenoid vacuum valve control circuit and the problem still exists, replace the PCM with a known good unit and retest.