The throttle control module contains two throttle position (TP) sensors with a common 5 volt supply, a common ground, and two independent signal circuits that vary resistance according to changes in throttle plate position. The ECM measures the signal voltage from each TP sensor, which is a reduced value of the ECM 5 volt reference. As the throttle plate opens from 0% throttle to wide open throttle, the voltage on the TP sensor 1 signal increases from 0.6 (+/-0.2) volts to 4.3 (+/-0.2) volts, while the voltage on the TP sensor 2 signal decreases from 4.4 (+/-0.2) volts to 0.76 (+/-0.2) volts. DTC P0121 sets when the TP sensor 1 indicates too large of a difference in throttle angle based on the mass air flow (MAF) reading.

DTC P0121 will set if TP sensor 1 percentage differs greater than 13% based on the MAF sensor signal when:

DTC P0121 diagnostic runs continuously with engine running.

P0121 is a (type A) DTC.

DTC P0121 indicates TP sensor 1 conflicts with the MAF reading.

A skewed TP sensor 1 can set this DTC. Check the voltage range of the sensor using THROTTLE POSITION VALVE in the SPECIAL FUNCTIONS section of the Scan tool, which will command the throttle control module to move the throttle plate. The approximate (+/- .1) TP SENSOR 1 voltages at 25% TP ANGLE intervals should be:

If there is a circuit fault detected for TP sensor 1 and TP sensor 2 in the same ignition cycle, the ECM will default to REDUCED PERFORMANCE mode for the entire ignition cycle, even if the fault is corrected.

Possible causes: