Powertrain Controls - ECM/PCM

Throttle Position Sensor Circuit Code 21:

Throttle Position Sensor 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 .5 volt at idle to about 4.0 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 control outputs. On nonadjustable TPS switches, each time voltage drops below 1.25 volts and stops, the ECM assumes this value is 0 throttle angle and measures percent throttle from this point on.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

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

- Engine running.
- TPS signal voltage is greater than about 2.5 volts.
- 2.8L MAP sensor less than 2 volts.
- All conditions met for 8 seconds. With throttle closed, the TPS should read less than 1.25 volts. Refer to "Diagnostic Aids."

2. With the TPS sensor disconnected, the TPS voltage should go low if the ECM and wiring is OK.

3. Probing CKT 452 with a test light checks the 5 volt return circuit. This step isolates a faulty sensor ECM or an open CKT 452.


Diagnostic Aids:
A Tech 1 "Scan" tool reads throttle position in volts. Should read about .45 to .95 volt with throttle closed and ignition "ON" or at idle. Voltage should increase at a steady rate as throttle is moved toward Wide Open Throttle (WOT).
Also, some Tech 1 "Scan" tools will read throttle angle .0 % = closed throttle 100 % = WOT.
Scan TP signal while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from below 1.25 volts (1250 mV) when throttle was closed, to over 4.5 volts (4500 mV) when throttle is held at Wide Open Throttle (WOT) position.
A Code 21 will result if CKT 452 is open or CKT 417 is shorted to voltage.
SE.Intermittents