P0121: OBD-II code P0121 means the throttle position sensor A signal is present but doesn't correlate with expected throttle movement — usually a worn sensor track or dirty throttle body.

Symptoms

• Check engine light is on; the vehicle may enter reduced-power (limp) mode intermittently on some models.
• Erratic idle, hesitation, or dead spots when opening the throttle from rest.
• Cruise control may drop out or refuse to engage because the PCM can't trust the throttle input.

Common causes

• A worn TPS with dropouts or dead spots in its resistive track — common on higher-mileage cable-throttle engines.
• A dirty throttle body plate not returning to the same rest position each time.
• A miscalibrated APP sensor after a battery disconnect without a throttle relearn on drive-by-wire engines.

What to do?

Read live TPS voltage while slowly sweeping the pedal — voltage should rise smoothly with no jumps or dead spots. Clean the throttle body. Perform a throttle relearn with a scan tool after cleaning. Replace the sensor if the voltage sweep is uneven.