Summary
P0123 means the throttle position sensor (TPS) voltage has exceeded the maximum expected threshold — typically above 4.8V. The most common causes are a wiring short to voltage (35%), a failed TPS (30%), and connector corrosion (20%). The engine will enter limp mode or may idle high. This is the mirror of P0122 (low input). Check wiring and the TPS connector first — corrosion between signal and reference pins is a common, cheap fix.
Severity: Moderate to High — PCM may interpret as wide-open throttle
Safe to drive: No — potential for high idle or unintended acceleration
Repair cost: $5–$400 depending on cause
DIY difficulty: Easy to Moderate
What does P0123 mean?
The TPS sends a voltage signal proportional to throttle opening. At closed throttle, it reads approximately 0.5V; at wide-open throttle, approximately 4.5V. P0123 sets when the signal exceeds the PCM's upper limit — usually above 4.8V — even when the throttle should be closed or partially open.
The PCM sees this as either the throttle being wide open when it shouldn't be, or as a circuit fault. On cable-throttle vehicles, the PCM may increase idle speed dramatically or flood the engine with fuel. On electronic throttle body vehicles, the PCM enters reduced power mode as a safety measure — it can't trust the throttle input and restricts engine output.
If the signal is stuck high, the PCM may also disable cruise control and traction control systems that depend on accurate throttle position data.