P0400

Wiring Diagram:

Circuit Description
The exhaust gas recirculation (EGR) system controls the formation of NOx (Oxides of Nitrogen) emissions by recirculating the exhaust gases into the combustion chamber. The powertrain control module (PCM) uses the expected changes in the manifold absolute pressure (MAP) sensor readings in order to evaluate the performance of the EGR system. The PCM applies full manifold vacuum to the EGR valve while monitoring the MAP sensor signal. The PCM suspects that the EGR valve flow is incorrect when the monitored MAP sensor pressure change is either too large or too small.

Conditions for Setting the DTC
Engine coolant temperature is between 75°C and 110°C (167°F and 230°F).
^ Intake air temperature between -7°C and 70°C (19°F and 158°F).
^ Barometric pressure greater than or equal to 75 kPa.
Intake pressure change during steady state driving conditions:
^ Engine speed between:
- 1500 and 3000 RPM (A/T)
- 2300 and 4000 RPM (M/T)
^ Vehicle speed greater than 32 km/h (20 mph).
^ Throttle position change is less than 0.37 / 5 msec.
^ 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:
^ Engine speed between 1500 and 4000 RPM.
^ Engine speed change less than 150 RPM / 25 msec.
^ Vehicle speed greater than 46 km/h (28 mph).
^ After 290 seconds from engine start in fuel shut-off mode.
^ Intake pressure difference diagnostic will run for 2 seconds, once per drive cycle.

Action Taken When the DTC Sets
^ The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
^ The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
^ The MIL turns OFF after three consecutively passing trips without a fault present.
^ A History DTC clears after 40 consecutive warm-up cycles without a fault.
^ Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

Diagnostic Aids
Normal engine operating temperature is specified as 80°C (176°F) or above.
Check for any of the following conditions:
^ A faulty vacuum hose connection, leakage, deterioration, blockage or incorrect routing.
^ An EGR valve that is stuck closed, stuck open, or binding.
^ An EGR pressure transducer malfunction.
^ An EGR solenoid vacuum valve or an EGR bypass valve malfunction.
^ 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.
^ A faulty electrical connection at the PCM.
Perform functional checks of the individual EGR system components. Refer to EGR System Diagnosis.

A DTC P0400 may set when the expected change in MAP is greater than or less than the expected value, during the operation of the EGR bypass valve.
Investigate any condition that can affect the accuracy of the MAP sensor readings.
^ Retarded ignition timing.
^ Restricted exhaust system.
^ Faulty vacuum supply hoses to the MAP sensor.
An intermittent malfunction may be caused by a fault in the EGR control system electrical circuits. Inspect the wiring harness and components for any of the following conditions:
^ Backed out terminals.
^ Improper mating of terminals.
^ Broken electrical connector locks.
^ Improperly formed or damaged terminals.
^ Faulty terminal to wire connections.
^ Physical damage to the wiring harness.
^ A broken wire inside the insulation.
^ Corrosion of electrical connections, splices, or terminals.
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.

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. This step checks for an EGR valve that is staying open.
3. This step determines if there are faults in the EGR valve or faults in the EGR bypass valve and components. The EGR bypass valve output control is disabled by the PCM until the engine has reached a normal operating temperature of at least 80°C (176°F) and all DTCs are cleared.
4. This step determines if fault is present in the EGR pressure transducer or the EGR solenoid vacuum valve components. The EGR valve should not operate on a cold engine when the RPM is increased. Bring the engine to normal operating temperatures and increase the RPM. The EGR valve should move accordingly.
5. Driving the vehicle under these conditions will verify that the fault is present. The fault is intermittent if no DTC sets.
9. This step physically checks for the cause of an incorrect flow of exhaust gas.
14. This step checks the EGR bypass valve control circuit for an open to the PCM. If the circuit checks OK the PCM may be faulty. Replace the PCM with a known good unit and retest the vehicle.
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 (17°F) and all DTCs are cleared.
22. This step checks the EGR solenoid vacuum valve control circuit for an open or a short. If the circuit checks OK the PCM may be faulty. Replace the PCM with a known good unit and retest the vehicle.