The Throttle Position Sensor (tps) is connected to the throttle shaft and is controlled by the throttle mechanism. A 5 volt reference signal is sent to the tps from the ecm. As the throttle valve angle is changed (accelerator pedal moved), the resistance of the tps also changes. At a closed throttle position, the resistance of the tps is high, so the output voltage to the ecm will be low (approximately 0.5 volt). As the throttle plate opens, the resistance decreases so that, at wide open throttle, the output voltage should be approximately 5 volts. At closed throttle position, the voltage at the tps should be less than 1.25 volts.
By monitoring the output voltage from the tps, the ecm can determine fuel delivery based on throttle valve angle (driver demand). The tps can either be misadjusted, shorted, open or loose. Misadjustment might result in poor idle or poor wide-open throttle performance. An open tps signals the ecm that the throttle is always closed, resulting in poor performance. This usually sets a Code 22. A shorted tps gives the ecm a constant wide-open throttle signal and should set a Code 21. A loose tps indicates to the ecm that the throttle is moving. This causes intermittent bursts of fuel from the injector and an unstable idle.