Federal Emissions

Code 21 Diagnostic Chart:

Throttle Position Sensor (TPS) Circuit Diagram:

CIRCUIT DESCRIPTION
The Throttle Position (TP) sensor provides a voltage signal that changes relative to the throttle blade. Signal voltage will vary from about .5 at idle to about 4.8 volts at Wide Open Throttle (WOT).

The TP sensor signal is one of the most important inputs used by the ECM for fuel control for many of the ECM control outputs.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 21 will set if:
^ No DTC 33 or DTC 34.
^ Engine is running
^ TP sensor signal voltage is greater than 3.8 volts.
^ Air flow is less than 17 gm/sec.
^ All conditions met for 10 seconds.
OR
^ With throttle closed. the TP sensor should read less than .98 volt. If it doesn't, make sure cruise control and throttle cables are not being held open.

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

3. Probing CKT 808 with a test light checks the 5 volt return circuit. Faulty sensor ground circuit will cause a DTC 21.

DIAGNOSTIC AIDS:

^ A Tech 1 (or equivalent) scan tool reads throttle position in volts. Voltage should increase at a steady rate as throttle is moved toward WOT.

^ Also some scan tools will read: throttle angle 0% = closed throttle, 100% = WOT.

^ An open in CKT 808 will result in a DTC 21.

^ DTCs 15 and 21 stored at the same time could be the result of an open CKT 808 which would also turn the temperature warning indicator "ON." Scan the TP sensor while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from about 500 mV (.5 volt) when throttle was closed, to about 4800 mV (4.8 volts) when throttle is held at Wide Open Throttle (WOT) position.