Throttle position sensor code 21 is a GM designation for cars manufactured before 1996. It is one of the easiest codes to diagnose and repair. Code 21 states that the input voltage from the sensor is too high when compared against normal values. The voltage supplied to the sensor should vary, depending on throttle operation by the driver. With sensor code 21, the voltage stays high all the time and is not in the normal range for the vehicle.
Raise the hood of the vehicle, remove the air cleaner and notice the way the throttle position sensor is connected to the throttle body. It is usually located just opposite the throttle cable and has three wires connected. Use the connector release clip as the orientation point. Looking down at the clip, the right wire is the voltage supply, the center is the output to the computer, and the left wire is the reference to ground, or the negative circuit.
Unplug the sensor and measure the far right wire by touching the positive lead of the meter to the metal terminal and the negative lead to the engine block or battery. Turn the ignition key and check the voltage reading. It should read 5 volts. If it reads significantly higher, the computer is defective. Use a socket wrench to remove the computer, usually inside the car, and retest the circuit.
Inspect the wire connector if the voltage reading is normal before replacing the sensor. If the voltage input is normal, the sensor internal contacts may be glazed or shorted causing a high reading, however, the sensor wire insulation could also be old and deteriorated. If the output wire is touching the input wire and the insulation is missing, the two copper wires short out, or are short of completing the circuit through the sensor. This causes the code to set.
Repair the wires or replace the sensor. Throttle sensor connectors are readily available. Use wire crimps to install the new connector. Unbolt the old sensor and install a new one using a 1/4-inch socket wrench.
Clip wire probes over the center and left wires that pierce the insulation. Turn the ignition key and look at the output voltage. When the throttle is closed, slightly adjust the sensor output to .05 volts and tighten the mounting bolts.
