DTC 21

Code 21 "Throttle Position Sensor":

Wiring Diagram - Throttle Position Sensor:

CODE 21

THROTTLE POSITION SENSOR (TPS) CIRCUIT (SIGNAL VOLTAGE HIGH)


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 .4 volt at idle to about 4.9 volts at wide open throttle. 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. Code 21 will set if:
TPS signal voltage is above 1.1 volts.
Engine speed is less than 750 rpm.
Pressure sensor indicates high manifold vacuum (closed throttle).
All conditions met for 10 seconds.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Checks CKTs 417 & 452 from the TPS connector back to the ECM. Installing a jumper between harness terminals "B" and "C" should pull voltage at ECM terminal "2" down to well below 2.5 volts.

2. Test light should be "ON" if ground CKT 452 is OK.

3. Test light "ON" indicates CKT 417 could be open, connection at ECM faulty or faulty ECM.

Diagnostic Aids:

A "Scan" tool displays TPS input voltage as an indication of throttle position. TPS voltage should be .31 to .41 volt with throttle closed, ILC retracted and ignition "ON." TPS voltage is normally between .475 and .625 volt with engine at warm idle. However, the ECM "normalizes" or "learns" the idle TPS voltage, so that an idle TPS voltage between .2 volt and .7 volt is acceptable and need not be adjusted. TPS voltage should increase at a steady rate as the throttle is opened and should reach at least 4.5 volts at wide open throttle (WOT). Also, some "Scan" tools will display throttle position as an angle. About 10% = closed throttle, 100% = WOT.


An open in CKTs 452 or 417 will result in a Code 21 due to 510k pull-up resistor inside ECM. Refer to "Intermittents" in DIAGNOSIS BY SYMPTOM - NO TROUBLE CODE STORED. Check For:

Poor Connection or Damaged Harness - Inspect ECM harness connectors for backed out terminals "2" or "22", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.

Intermittent Test - If connections and harness check OK, monitor TPS voltage display while moving related connectors and wiring harness. If the failure is induced, the "Scan" TPS voltage display will change. This may help to isolate the location of the malfunction.

Skewed Pressure Sensor - A false indication of high manifold vacuum from the pressure sensor could cause a false Code 21 if it occurred while throttle was open and engine speed below 750 rpm for 10 seconds. Chart C-1D provides a check for the pressure sensor. Also make sure the vacuum hose or pressure sensor are not trapping vacuum.