P048

Circuit Description

This diagnostic test is designed to check for an EGR system fault. After the engine is fully warmed up and the EGR system has been operating on full for a period of time, the PCM will disable the EGR valve and monitor the change in rear short term fuel trim for the next 5 seconds. If the change is 14 counts or greater, the EGR system is functioning normally and will not be retested for the current ignition cycle. If the change is less than 14 counts, the test is failed and the counter is incremented. When the counter reaches 4, Code P048 is set. If at any time before the counter reaches 4 a change in the rear short term fuel trim is 14 counts or greater the counter is decremented. If the counter reaches 0, the next rear short term fuel trim change of 14 counts or greater will pass the test for the entire ignition cycle. When Code P048 is set, either the EGR system was turned 'OFF' before the test started or the EGR system is functioning and the PCM does not have the ability to turn it 'OFF'.

Test Condition(s)

1. EGR has not passed for the entire ignition cycle (see CIRCUIT DESCRIPTION)
2. Codes P013, P014, P015, P017, P021, P022, P024, P031, P032, P034, P042, P043, P044, P045, and P075 not set.
3. 9.2 minutes have passed since closed loop fuel control was enabled.
4. Engine coolant temperature between 80°C and 110°C.
5. MAP between 50.2 kPa and 80 kPa.
6. Engine speed between 1250 rpm and 1850 rpm.
7. Vehicle speed 35 mph or greater.
8. Ignition voltage 11 volts or greater.
9. The following conditions present for 5 seconds prior to the test:
- EGR system on PULL
- Rear heated oxygen sensor toggles 5 times per second
- Rear short term fuel trim is 135 counts or less

Failure Condition(s)

1. PCM turns 'OFF' the EGR system for 5 seconds.
2. PCM monitors the rear short term fuel trim for a change.
3. Rear short term fuel trim change less than 14 counts; increment counter.
4. Counter reaches 4; set Code P048.

Action Taken

1. PCM turns 'ON' 'SERVICE ENGINE SOON' MIL
2. PCM disables EGR system.
3. Transaxle shift adapts are maintained at current levels.

Notes on Fault Tree

1. Diagnose Code P036 first because it can cause the EGR system to malfunction.
2. If rear bank long term fuel trim is 15 counts or greater less than front bank long arm fuel trim, there is a fuel control problem and use P046 to diagnose problem.
3. Checking for a rich engine condition that could have caused P048 to set.
4. Checking if by using PCM overrides the EGR valve can be shut off (less than 100 counts).
5. Checking if by using PCM overrides the EGR valve can returned to full on (greater than 200 counts).
6. Checking CKT 750 and fuse 'D1' for an open.
7. Checking if EGR valve is open circuited.
8. Checking if CKT 435 is open.
9. Checking if PCM is open internally or if CKT 435 is shorted to voltage.
10. EGR valve control circuits are OK. Checking to see if the EGR valve can respond correctly to the signal being sent to it from the PCM.