The 2021 Acura MDX's P0121 code indicates the Throttle Position Sensor's output is outside expected range for current conditions. Unlike circuit codes, P0121 means the TPS is communicating but values don't correlate with other parameters like accelerator pedal position or engine load.
Understanding P0121
P0121 indicates a correlation error—the TPS reading doesn't match expected values. For example, if the accelerator pedal is released but TPS shows partial throttle, or if full throttle is commanded but TPS shows only slight opening, these mismatches trigger P0121.
Electronic Throttle on the MDX
The 2021 MDX's 3.5L V6 or 3.0L turbo V6 uses electronic throttle control. The system compares TPS readings with accelerator pedal position, engine RPM, and calculated airflow to verify correct operation. Any significant discrepancy triggers performance codes.
Common Causes of P0121
A dirty throttle body can cause the throttle blade to stick or move erratically. Carbon buildup on the bore and blade prevents smooth operation, causing position readings that don't match commanded position.
A failing TPS may provide readings that don't accurately represent actual throttle position. Worn potentiometer tracks develop flat spots or erratic areas that cause inconsistent output.
Intake manifold vacuum leaks can affect throttle response in ways the ECU interprets as TPS problems. Air entering downstream of the throttle changes how the engine responds to throttle position.
Diagnostic Approach
Compare TPS position to commanded throttle position (accelerator pedal sensor output). They should closely correlate during normal operation. Monitor for smooth TPS signal changes during slow throttle sweeps—any hesitation, jumps, or flat spots indicate sensor issues. Inspect the throttle body for carbon buildup.