Parts
$20–90
P0121 — Throttle Position Sensor Range / Performance
CAUTIONIs it safe to drive? Driveable but erratic throttle response can make the car unpredictable, especially at highway speeds — fix it before extended driving.
What P0121 means
The TPS signal is within the normal voltage range but doesn't correlate correctly with MAP sensor readings or engine RPM. The sensor works but its output is inconsistent or non-linear through the throttle sweep.
Most likely causes (in order)
- TPS sensor worn at the idle or WOT ends of its sweep (most common — internal carbon track wears out)
- Throttle body dirty causing TPS to read correctly but actual airflow to be wrong
- Loose or intermittent TPS connector
- Vacuum leak making MAP/TPS mismatch
- Throttle cable binding or sticky (cable-actuated systems)
Symptoms you might notice
- Jerky or hesitant acceleration
- Erratic idle — may surge or hunt
- Unexpected power loss at cruise
- Transmission shift quality issues (automatic transmissions use TPS for shift points)
- Hard starting
What to check first
Watch live TPS percentage on a scan tool while slowly sweeping the throttle from idle to WOT — the reading should climb smoothly and linearly with no dropouts, spikes, or flat spots. Any glitch in the sweep means the TPS internal track is worn and the sensor needs replacement regardless of what it reads at rest.
Repair cost & difficulty
Labor
0.3–0.7 hr
Difficulty
DIY-friendly