Trouble Code P0121

P0121 – Throttle position (TP) sensor A/accelerator pedal position (APP) sensor / switch A -range/performance problem
The throttle position sensor (TPS) is located on the throttle body. It is a type of variable resistor which changes its resistance value according to throttle position. As the throttle is opened, the voltage reading goes up. This information is used by the PCM to determine output controls such as fuel injection and idle air control.

Most throttle position sensors are simple and easy to troubleshoot. There are three wires attached to the TPS: power, ground and signal. Power, or reference voltage, goes into the TPS. The sensor then sends a return signal voltage to the PCM. The return voltage depends on throttle positon. Typically, the signal value at idle will be about .45 volts. This voltage reading increases as the throttle is opened and will read about 4.5 volts at wide open throttle (WOT).

Many late model vehicles don’t use a throttle wire. Instead, they use what is referred to as “drive by wire”. In these systems, a motor is used to operate the throttle using inputs from position sensors on the accelerator pedal. When the gas pedal is depressed, the control module commands the throttle to open. Then, a pair of TPS sensors relay the throttle positon to the control module so it knows everything is working properly.