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Code Chart 21:

TPS Circuit:

CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to the throttle blade angle. Signal voltage will vary from about 0.33 to 1.33 volts at idle, to about 4.5 volts at Wide Open Throttle (WOT). The TPS signal is one of the most important inputs used by the ECM for fuel control, and for most of the ECM controlled outputs.

TEST DESCRIPTION: The numbers below refer to circled numbers on the diagnostic chart.

1. A Code 21 will set under the following conditions:

a. Engine idling.
b. TPS reading is less than .2 volt or greater than 2.5 volts.
c. MAP reading below 65 kPa.
d. All the above conditions are present for at least five seconds.
e. The TPS has an auto zeroing feature. If the voltage reading is within the range of about 0.33 to 1.33 volts, the ECM will use that value as closed throttle. If the voltage reading is out of the auto zero range at closed throttle, check for a binding throttle cable or damaged linkage. If OK, continue with diagnosis.

2. If the ECM recognizes the change of state, the ECM and circuits 474 and 417 are OK.

3. This step isolates a faulty sensor, ECM, or an open circuit 452. If circuit 452 is open, there may also be other codes stored.

DIAGNOSTIC AIDS:

^ A "Scan" tool displays TPS in volts. Closed throttle voltage should be between 0.33 to 1.33 volts, and the voltage value should increase at a steady rate as the throttle is opened.
^ If a Code 21 is intermittent, Testing and Inspection