Code 21, Throttle Position Sensor (TPS) & Idle Switch Circuit

TPS provides a voltage signal that changes relative to throttle opening. Signal voltage varies from less than .80 volt at idle to approximately 4.5 volts at wide open throttle. TPS signal is one of the most important inputs used by the ECM for fuel control and most ECM controlled outputs.

NOTE: Test numbers refer to test numbers on diagnostic chart.

This test determines if idle switch portion of circuit is faulty.
This checks if Code 21 is from a hard failure or an intermittent condition. Code 21 will set if both Wide Open Throttle (WOT) and idle are detected for more than one second or TPS is in off-idle or WOT position with engine speed between 500-600 RPM with these conditions present for 32 seconds.
If ECM sees change of state, ECM and circuits No. 416 and 417 are okay.
This will isolate a faulty sensor, ECM or an open in circuit No. 452.