Engine Control Module ECM

DTC 21 Chart:

DTC 21 Circuit:

Circuit Description:
The Throttle Position (TP) sensor provides a voltage signal which 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 TP signal is one of the most important inputs used by the ECM for fuel control and for most of the ECM control outputs.

On non adjustable TP 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.

DTC 21 Will Set When:

^ Engine running.

^ TP signal voltage is greater than 2.5 volts.

^ All conditions met for eight seconds. With throttle closed, the TP signal should read less than 1.25 volts. Refer to "Diagnostic Aids.

Action Taken (ECM will default to): The Malfunction Indicator Lamp (MIL) will illuminate.

DTC 21 Will Clear When: Conditions for fault are no longer present and Tech 1 "Clear DTCs" function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and ECM B fuse is removed for 10 seconds.

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

1. This step checks to see if DTC 21 is the result of hard failure or an intermittent condition.

2. With the TP sensor disconnected, the TP signal 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 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 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 DTC 21 will result if CKT 452 is open or CKT 417 is shorted to voltage.

Refer to DIAGNOSIS BY SYMPTOM/ INTERMITTENTS - Intermittent Malfunctions.