DTC 21

Fig. 18 Throttle position sensor wiring circuit:

If this code is not displayed during the third pass of diagnostic codes, the problem is an intermittent failure. To help isolate the problem, manipulate related wiring while observing parameter .0.1. If the failure is induced, the reading will jump from its normal value to a shorted reading of greater than 77° at part throttle.
The ECM supplies a 5 volt signal to pin C of throttle position sensor and supplies a ground path to pin B. The TPS signal from pin A is a variable voltage, dependent on throttle position. Code 21 indicates that the voltage signal from TPS is too high. This situation can be observed if the value of parameter .0.1 is greater than 77°. Possible causes of this condition are:
a. Defective throttle position sensor.
b. Defective wiring harness or electrical connectors.
c. ECM unable to properly process signals.

To diagnose a Code 21, proceed as follows:
1. Turn ignition on, enter diagnostics, and display parameter .0.1, then disconnect TPS connector.
2. If value of parameter .0.1 is (-)10° to (-)7°, check voltage between circuits 474 and 476-X.
3. If no voltage is observed, check circuit 476-X for an open and repair as necessary. If voltage observed is 5 volts, check mating of sensor terminals and operation of TPS and replace or repair as necessary.
4. If value of parameter .0.1 is (-)6 or more in step 2, check circuit 417 for a short to voltage and repair as necessary. If circuit is satisfactory, check for faulty ECM or ECM connector and repair or replace as necessary.