P0120 Code: Throttle/Pedal Position Sensor A Circuit
The throttle position signal dropped out or glitched, so the PCM no longer trusts the throttle angle.
At a glance
Severity: Moderate
- Meaning
- Throttle/Pedal Position Sensor A Circuit
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$850
- DIY difficulty
- Moderate
The throttle position sensor (TPS) is fed 5 volts and returns a signal that rises steadily as the throttle opens, typically from about 0.5 volts at idle to around 4.5 volts at wide open. P0120 is the general circuit fault for sensor A: the PCM saw the signal behave in a way no throttle can, such as a sudden dropout, a spike or a value that jumped around, without necessarily staying high or low long enough to set P0122 or P0123. Older engines with a throttle cable and a bolt-on potentiometer are the typical candidates, because the carbon track inside wears at the idle and light-cruise positions where it spends most of its life. On drive-by-wire engines the sensor is built into the throttle body, and the same fault usually puts the car into reduced-power limp mode.
Can I keep driving? Drive gently and only as far as needed. A glitching throttle signal can cause sudden stumbles, surges or stalling when you lift off, and on electronic throttles the car may drop into limp mode without warning in traffic.
How to read P0120
- P Powertrain
- 0 Generic (SAE)
- 1 Fuel & air metering
- 20 Specific fault
Symptoms
- Check engine light on, sometimes along with a reduced-power or wrench light
- Stumble or hesitation at a specific pedal position, often just off idle
- Idle speed hunting up and down, or stalling when coming to a stop
- Automatic transmission shifting too early, too late or harshly
- Engine stuck at a fast idle with little response to the pedal (electronic throttle limp mode)
- Symptoms that come and go over bumps or as the engine bay heats up
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Worn spot on the sensor's resistive track causing momentary signal dropouts
- Loose, corroded or water-contaminated TPS connector
- Intermittent break or chafe in the signal, reference or ground wire
- Throttle body with integrated sensor failing (drive-by-wire engines)
- Problem on the shared 5-volt reference circuit, often with other sensor codes present
- PCM input fault or incorrect replacement sensor
Diagnosis step by step
Record freeze frame and look for companion codes
Save the freeze frame so you know the rpm, load and throttle angle when it failed. If P0122 or P0123, a pedal sensor code or several 5-volt sensor codes are also stored, follow that lead first: a shared reference fault or a throttle body failure shows up that way.
Tools: OBD-II scanner
Sweep the throttle while graphing the signal
Key on, engine off, graph the throttle position voltage or percentage and open the throttle slowly by hand from closed to wide open and back. The trace must rise and fall smoothly with no notches, flat spots or drops toward zero. A scan tool may be too slow to catch a brief dropout; an analog-style meter bar graph or a lab scope is better.
Tools: OBD-II scanner, Multimeter
Wiggle-test the connector and harness
With the signal still displayed, hold the throttle at a steady position and flex the connector and the wiring loom back toward the main harness. Any flicker in the reading pinpoints a poor terminal or a broken strand. Look for green corrosion and for a connector lock tab that no longer holds.
Tools: OBD-II scanner, Flashlight
Check reference and ground at the sensor
Back-probe the three wires with the key on. Reference should read 4.9 to 5.1 volts and ground should be under about 0.1 volt to battery negative. If the reference dips or wanders, unplug other 5-volt sensors one at a time to find one that is loading it down.
Tools: Multimeter
Check closed-throttle voltage and adjustment
On older cable-throttle engines with slotted TPS mounting holes, closed-throttle voltage is usually specified somewhere around 0.45 to 0.9 volts. Make sure the throttle plate actually closes fully and the cable is not holding it open. Drive-by-wire sensors are not adjustable, so skip this there.
Tools: Multimeter, Screwdriver
Replace the sensor or throttle body and relearn
Replace the bolt-on TPS, or the complete throttle body on drive-by-wire engines, and clear the codes. Many vehicles then need an idle or throttle relearn, done with a scan tool or by a specific key-on and idle routine, before idle quality returns to normal.
Tools: Screwdriver, OBD-II scanner
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean and secure TPS connector, apply dielectric grease | Yes | $0–$60 |
| Replace bolt-on throttle position sensor (cable throttle) | Yes | $90–$260 |
| Replace electronic throttle body with integrated sensor | No | $300–$850 |
| Repair TPS wiring or 5-volt reference circuit | No | $100–$350 |
| Idle/throttle relearn after repair | Yes | $0–$100 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
P0120 turns on the check engine light, which fails an OBD-II emissions inspection. Make sure the light stays off and the monitors show ready before going back for the test.
Frequently asked questions
What's the difference between P0120 and P0122 or P0123?
P0122 and P0123 mean the signal was held too low or too high. P0120 is the broader circuit fault, used by some PCMs for erratic or intermittent signals and by older ones for any TPS circuit problem. It often points to a worn sensor or loose connection rather than a wire shorted solid.
Why does my automatic shift strangely with this code?
The transmission uses throttle position to decide shift points and line pressure. With a glitching or substituted signal it may upshift too early, hold gears too long or shift hard, and some vehicles lock in a fixed gear until the fault clears.
Can I clean the throttle body to fix P0120?
Cleaning removes carbon that affects idle, but it doesn't repair a worn sensor or a bad connection. Keep spray cleaner away from the sensor and its connector, because solvent that gets inside can make the fault worse.
Does a new TPS need to be adjusted?
Only on older engines with slotted mounting holes; set it to the closed-throttle voltage from the service information. Most newer sensors are fixed in place, and on drive-by-wire engines the PCM learns the closed position itself during a relearn.