OBD-II trouble code
P0120: Throttle/Pedal Position Sensor/Switch "A" Circuit Malfunction
The engine computer has detected a fault in the circuit for throttle position sensor A — the sensor that tells it how far the throttle is open, or on drive-by-wire vehicles, how far you've pressed the pedal. Because throttle control depends on this signal, the car may hesitate, idle badly, or drop into reduced-power mode.
Quick facts
- System
- Powertrain
- Category
- Throttle / Idle
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $700
- DIY difficulty
- Intermediate DIY
What does P0120 mean?
P0120 sets when the ECM sees a problem in the throttle position sensor (TPS) 'A' circuit — the signal is missing, stuck, or outside the voltage window the manufacturer allows. The TPS is a position sensor on the throttle body (and on drive-by-wire vehicles, a matching sensor set on the accelerator pedal) that converts throttle opening into a voltage the computer reads continuously. It is one of the most important inputs the ECM has: fuel delivery, ignition timing, transmission shift decisions, and idle control all lean on knowing exactly how far the throttle is open.
P0120 is the general 'circuit malfunction' code for sensor A, distinct from its siblings: P0121 flags a signal that's implausible for the conditions, while P0122 and P0123 flag voltage stuck low or high. P0120 casts the widest net — an open or short anywhere in the circuit, a bad connection, a failing sensor element, or a reference-voltage problem can all land here. Modern throttle position sensors are typically non-serviceable and inexpensive relative to the drivability trouble they cause, but the wiring deserves attention first, because a damaged connector or chafed harness produces exactly the same code as a dead sensor.
Because an untrusted throttle signal is a safety issue, many vehicles respond to P0120 aggressively: the ECM may ignore the sensor, force a fixed idle strategy, or cut power dramatically ('limp mode') to keep the car controllable. That's why this code rates high severity even though the car usually still moves — a vehicle that hesitates or surges when you press the pedal is a real hazard in traffic. On drive-by-wire vehicles the throttle body and pedal each have redundant sensors, and a correlation code such as P2135 stored alongside P0120 strengthens the case for a genuine sensor or wiring fault rather than a passing glitch.
Common causes
- Failed throttle position sensor (often part of the throttle body assembly)
- Open or shorted TPS signal, ground, or 5-volt reference wiring
- Corroded, loose, or damaged TPS connector
- Carbon buildup causing erratic throttle plate movement
- Worn throttle body causing an unstable sensor reading
- Failing accelerator pedal position sensor on drive-by-wire vehicles
- ECM fault (rare — verify sensor and wiring first)
Symptoms
- Check engine light on with P0120 stored
- Hesitation, stumble, or surging when accelerating
- Unstable, high, or low idle
- Stalling, especially at stops
- Reduced power / limp mode, sometimes capping speed
- Harsh or mistimed transmission shifts
- Poor fuel economy
Diagnostic steps
- 1.Scan for companion codes — P0121, P0122, P0123, or a correlation code like P2135 narrows the fault considerably.
- 2.Inspect the TPS connector and wiring at the throttle body for damage, corrosion, and loose terminals.
- 3.Back-probe the sensor and watch the signal voltage sweep as the throttle opens — it should rise smoothly with no dropouts.
- 4.Verify the 5-volt reference and sensor ground are present and clean.
- 5.On drive-by-wire vehicles, check the accelerator pedal sensor data as well as the throttle body sensor.
- 6.If the sensor is part of the throttle body, inspect for carbon buildup and check the throttle plate for smooth movement before replacing the assembly.
Repair cost
$100 – $700
A standalone TPS is inexpensive — often $50-$150 in parts with modest labor. On most modern vehicles the sensor is integrated into the throttle body, pushing a replacement to roughly $300-$700 including labor, and some vehicles require a relearn procedure with a scan tool afterward. Wiring and connector repairs land at the low end. Throttle body cleaning is cheap and sometimes resolves erratic readings.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with throttle body cleaning preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.