With Speed Density Update

Chart - Code 21 Throttle Position Sensor (Signal Voltage High):

Throttle Position Sensor (TPS) Circuit:

CIRCUIT DESCRIPTION
The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to the throttle blade. Signal voltage will vary from about 0.29 at idle to about 4.8 volts at wide open throttle. The TPS signal is one of the most important inputs used by the Electronic Control Module (ECM) for fuel control and for most of the ECM control outputs.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. Code 21 will set if:
^ Engine is running.
^ The TPS signal voltage is greater than 3.75 volts.
^ Air flow is less than 17 GM/sec.
^ All conditions met for 10 seconds.
OR
^ TPS signal voltage over 4.8 volts with ignition "ON."
With throttle closed, the TPS should read less than 0.98 volt. If it doesn't, make sure cruise control and throttle cables are not being held open. Check TPS adjustment.
2. With the TPS sensor disconnected, the TPS voltage should go low if the ECM and wiring is OK.
3. Probing circuit 452 with a test light checks the sensor ground circuit, because a faulty sensor ground will cause a Code 21.

DIAGNOSTIC AIDS
A "SCAN" tool reads throttle position in volts. Voltage should increase at a steady rate as throttle is moved toward wide open throttle.
Also some "SCAN" tools will read throttle angle 0% = closed throttle 100% = wide open throttle. An open in circuit 452 will result in a Code 21.
Codes 15 and 21 are stored at the same time could be the result of an open circuit 452 which would also turn the temperature warning indicator "ON," "SCAN" TPS while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from 290 mV (0.29 volt) when throttle was closed, to over 4800 mV (4.8 volts) when throttle is held at wide open throttle position.

Refer to Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions