Diagnostic Aids

NOTE: If this code IS NOT displayed during the third pass of diagnostic codes, it is an intermittent failure and cannot be diagnosed using this procedure. For an intermittent code, refer to NOTE ON INTERMITTENTS.

The ECM supplies a 5 volt signal to pin C (circuit 474) of the Throttle Position Sensor (TPS) and supplies a ground path to pin B (circuit 476-X). The TPS signal from pin A (circuit 417) is a variable voltage which is dependent on the throttle position.

Trouble Code 21 indicates that the voltage signal from the TPS is too high. This situation can be observed in engine data display as a "throttle angle" reading (parameter .0.1) of greater than 77 degrees at part throttle. Possible causes of this condition are:

A. Defective throttle position sensor.
B. Defective wiring, terminals, etc.
C. An ECM which is unable to process signals properly.

If the opposite condition (low TPS signal) can be created, then the self-diagnostics can be used to detect this failure by displaying engine data parameter .0.1. With the opposite condition created, parameter .0.1 should display a throttle angle" reading of 10 to -7 degrees. To create a low sensor signal, disconnect the TPS connector. If the "throttle angle" reading is -10 to -7 degrees circuit 476-X must be checked for an open. If the voltage between circuits 474 and 476-X is 0 volts circuit 476-X is open. If the voltage is 5 volts, the ECM and wiring are OK. The fault must be improper mating of sensor terminals or a faulty TPS sensor.
If the "throttle angle" reading is -6 degrees or greater, then there is a short to voltage in the wiring harness or the ECM.