P0121 — Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance Problem

What it means

The signal from throttle position sensor A stays within its electrical limits but does not agree with other engine data such as mass air flow, manifold pressure, engine speed or the second throttle sensor. The ECM concludes that the reported throttle angle is implausible rather than simply shorted or open. The driver usually notices flat spots during acceleration, jerky part-throttle response, wrong idle speed and harsh or delayed automatic transmission shifts.

Likely causes (most common first)

1. Dead spot or wear in the sensor track causing a jumpy signal
2. Carbon build-up on the throttle plate preventing it from closing fully
3. Throttle position not relearned after cleaning or replacing the throttle body
4. Intermittent break or high resistance in the signal wire or ground
5. Vacuum leak that makes measured air flow disagree with throttle angle
6. Mechanical wear in the throttle linkage or pedal module

Can you drive with it?

The car can still be driven carefully, but uneven power delivery makes it unsafe to postpone the repair for long.