P027

DTC P027:

Throttle Position Switch Circuit:

CIRCUIT DESCRIPTION:
The diagnostic test monitors the ISC throttle position switch during coast down conditions with the throttle position switch open and brake applied. At coast down, the PCM commands the ISC motor to retract and monitors the Throttle Position (TP) sensor for a corresponding decrease in throttle position. If the PCM detects a decrease in throttle position sensor reading two times in succession, then the PCM knows the throttle is resting on the ISC plunger and the throttle position switch should be closed and it sets Code P027.

TEST CONDITION(S):
1. Codes P021, P022 and P024 are not set.
2. Vehicle in coast down condition.
3. Throttle position switch open.
4. Brake applied.
5. Throttle position 12 degrees or less.
6. Engine speed at least 64 RPM greater than desired.
7. At least 10 seconds have passed between brake applications or the last ISC motor pulse from the PCM.

FAILURE CONDITION(S):
1. The PCM retracts the ISC motor and sees a corresponding decrease in throttle angle twice.

ACTION TAKEN:
1. PCM turns "ON" the "SERVICE ENGINE SOON" MIL.
2. PCM disables EGR.
3. Garage shift adapts are maintained at current levels.

NOTES ON FAULT TREE:
1. "LOW FAN" status light should cycle from "ON" to "OFF" to "ON" when accelerator is depressed and released.
2. Checking if fault is with the throttle position switch.
3. Checking CKT 450 for an open.
4. Checking PCM ability to recognize signal or if CKT 427 is open.

NOTE ON INTERMITTENTS:
Code P027 may be set if the Torque Converter Clutch (TCC) Brake Switch is sticking open or the accelerator and brake pedals are depressed simultaneously. Use input switch test P171 to determine if the brake switch is functioning properly.