Parts
$20–90
P0123 — Throttle Position Sensor Circuit High Voltage
CAUTIONIs it safe to drive? The ECU reads full throttle even at idle — expect rough running, possible stalling, and transmission issues on automatics; don't drive far until diagnosed.
What P0123 means
The TPS signal is stuck above 4.8–5V, which the ECU reads as wide-open throttle. It cannot distinguish idle from full acceleration, causing fueling and spark timing to be badly miscalculated.
Most likely causes (in order)
- Signal wire shorted to the 5V reference wire (most common)
- Open ground circuit at the TPS connector
- Failed TPS sensor (internal open or short to reference)
- Damaged wiring harness — chafed wires touching each other
- Faulty ECU (rare — rule everything else out first)
Symptoms you might notice
- High idle or surging
- Very rough or unstable idle
- Hesitation and stumble
- Automatic transmission shifts incorrectly — may stay in low gear
- Possible stalling
What to check first
Disconnect the TPS connector and measure the signal wire voltage at the harness side with key on — it should read 0V with the sensor unplugged. If it reads 5V with the sensor disconnected, the signal wire is contacting the 5V reference wire in the harness. Inspect closely for chafing where the harness passes near the throttle body.
Repair cost & difficulty
Labor
0.3–0.7 hr
Difficulty
DIY-friendly