DTC 21

Code 21 Diagnostic Chart:

Code 21 Circuit Diagram:

CIRCUIT DESCRIPTION:

The throttle Position Sensor (TPS) provides a voltage signal that changes relative to the throttle valve angle. Signal voltage will vary from less than 0.80 volts at idle to about 4.5 volts at wide open throttle (WOT). The idle switch provides the ECM with information used to decide closed loop operation and indicates the off-idle condition to the ECM. This switch is normally closed at idle and opens above 1800 rpm, which is then the off-idle condition. The TPS signal is one of the most important inputs used by the ECM for fuel control and for many of the ECM controlled outputs.

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

1. These tests will determine if the idle switch portion of the circuit is at fault when a Code 21 is set.

2. This step checks to see if Code 21 is the result of a hard failure or an intermittent condition. A Code 21 will set if:

^ Both WOT and idle are detected for more than one second. OR
^ ECM senses TPS in OFF-IDLE or WOT position and engine speed is between 500 and 600 rpm.
^ Above conditions are present for 32 seconds.

3. If the ECM recognizes the change of state, the ECM and CKTs 416 and 417 are OK.

4. This step isolates a faulty sensor, ECM or an open CKT 452.

DIAGNOSTIC AIDS:

A "Scan" tool displays throttle position in volts. Closed throttle voltage should be less than 0.80 volts. TPS voltage should increase at a steady rate as throttle is moved to WOT. A Code 21 will result if CKT 452 is open or CKT 417 is shorted to voltage. If Code 21 is intermittent, refer to DIAGNOSIS BY SYMPTOM under TESTING PROCEDURES.