The 2022 Lexus RX uses an electronic throttle system where the Throttle Position Sensor (TPS) communicates pedal position to the ECU. When P0120 appears, indicating a TPS circuit malfunction, the engine may enter reduced power mode for safety. Here's how to diagnose this important sensor issue.
What P0120 Means
P0120 is a general circuit malfunction code for the TPS. Modern vehicles use electronic throttle control (drive-by-wire), making the TPS critical for telling the ECU how far to open the throttle. A circuit malfunction means the ECU cannot reliably determine throttle position.
Electronic Throttle System Overview
The RX's electronic throttle uses multiple sensors for redundancy. The TPS typically consists of two potentiometers with opposite signal directions—as one increases, the other decreases. The ECU compares these signals to verify accuracy. A malfunction in either sensor or their circuits triggers P0120.
Common Causes of P0120
Wiring issues in the TPS circuit are common. The connector may be loose, corroded, or damaged. Wiring can chafe against components or develop breaks from vibration and heat cycling.
A failed throttle body can cause TPS issues since the sensors are integrated. Internal contamination, motor failure, or sensor degradation can all trigger P0120.
Reference voltage problems—if the 5V reference from the ECU is unstable—cause TPS signal issues. The TPS needs clean, stable reference voltage for accurate output.
Diagnostic Approach
Monitor TPS voltage with a scan tool while slowly moving the throttle. Voltage should increase smoothly from approximately 0.5V at idle to 4.5V at wide-open throttle. Any jumps, drops, or dead spots indicate sensor problems. Check both TPS signals if your scan tool shows them—they should mirror each other in opposite directions.