A throttle position problem can make your engine hesitate, surge or drop into reduced power mode. The check engine light comes on, and sometimes a wrench or “reduced power” light joins it. A P0120 code means the engine computer found a problem in the throttle position sensor “A” circuit.
This guide explains how the sensor works, what breaks the circuit and how to test it before you buy a throttle body.
P0120 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 20 | Specific fault | Throttle Position Sensor/Switch A Circuit Malfunction |
What Does the P0120 Code Mean?
The SAE J2012 standard defines the P0120 code as Throttle Position Sensor/Switch A Circuit Malfunction. The PCM found an electrical problem in the “A” throttle position sensor (TPS) circuit that does not fit a simple high or low reading.
The TPS reports how far the throttle plate is open. Most sensors use three wires: a 5-volt reference, a ground and a signal. On many engines, the signal reads about 0.5 to 1.0 volt with the throttle closed and rises to about 4.5 volts at wide-open throttle. Your service manual lists the exact values.
Older engines with a throttle cable use one sensor. Newer engines with electronic throttle control (drive-by-wire) use two sensors, “A” and “B,” built into the throttle body. The powertrain control module (PCM) compares them constantly and may cut power if they disagree.
Common Symptoms of a TPS Circuit Fault
The throttle reading affects fuel, timing, idle and shifting. Watch for these signs:
- Check engine light on
- Reduced power or limp mode, especially on drive-by-wire engines
- Hesitation or stumble when you press the gas
- Surging or unstable idle
- Stalling when you come to a stop
- Harsh or erratic automatic transmission shifts
What Causes the P0120 Code?
Wiring and connector faults cause many cases. The sensor itself wears out too. These are the usual causes:
- Loose or corroded connector. Heat and vibration loosen the throttle body connector.
- Damaged wiring. Wires can chafe or break near the throttle body.
- Worn TPS. The sensor’s internal track wears out, especially at the idle position.
- Shorted 5-volt reference. Many PCMs share one reference with other sensors, such as the MAP sensor. A short in any of them drags the reference down.
- Failed throttle body. On drive-by-wire engines, the TPS is part of the throttle body.
- PCM fault. This is rare, so test everything else first.
How to Diagnose the P0120 Code Step by Step
You need a scan tool with live data, a digital multimeter and a wiring diagram for your vehicle. Pin layouts vary, especially on drive-by-wire throttle bodies.
Check Live Data
Turn the key on without starting the engine. Watch the TPS reading in percent or volts as you press the gas pedal slowly to the floor. The reading should rise smoothly with no jumps or dropouts.
On drive-by-wire engines, the throttle opens only when the PCM commands it. Many scan tools show both TPS A and TPS B. The two should move together, often in opposite directions or at different scales, as your service manual describes.
Test the Three Wires
Back-probe the connector with the key on. Test each wire:
- Reference: Should read close to 5 volts. A low reading points to a short on the shared reference or a PCM problem.
- Ground: Should read close to 0 volts against the battery negative post.
- Signal: Should rise smoothly from about 0.5 volt to about 4.5 volts as the throttle opens.
If the reference reads low, unplug the other sensors on the same circuit one at a time. The reference returns to 5 volts when you unplug the shorted one.
How to Fix the P0120 Code
Match the repair to what your tests found. Then clear the code and road test the vehicle.
- Connector problems: Clean corroded pins or replace the pigtail.
- Wiring damage: Repair wires with solder and heat shrink tubing.
- Worn cable-throttle TPS: Replace the sensor. Some need adjusting to a set idle voltage.
- Drive-by-wire engines: Replace the throttle body assembly if the built-in sensor has failed.
- Shorted reference: Replace the sensor that drags the reference down.
Many drive-by-wire engines need a throttle relearn after a new throttle body or battery disconnect. Follow your service manual or use a scan tool with that function.
Repair Costs at a Glance
Costs depend a lot on whether your engine uses a separate sensor or a full electronic throttle body. This table shows typical ranges.
| Repair | DIY Parts Cost | Typical Shop Cost |
|---|---|---|
| Clean connector and pins | $10 to $15 | $50 to $100 |
| Wiring repair | $10 to $30 | $100 to $300 |
| Standalone TPS replacement | $25 to $100 | $100 to $250 |
| Electronic throttle body replacement | $150 to $600 | $300 to $900 |
Is It Safe to Drive with a P0120 Code?
You can often drive carefully to a shop, but the engine may lose power without warning. Drive-by-wire engines often enter reduced power mode, which limits speed and acceleration.
That loss of power can be dangerous when you merge or pass. Avoid highways if you can, and get the fault diagnosed quickly.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0121 | Throttle Position Sensor/Switch A Circuit Range/Performance Problem |
| P0122 | Throttle Position Sensor/Switch A Circuit Low Input |
| P0123 | Throttle Position Sensor/Switch A Circuit High Input |
| P0124 | Throttle Position Sensor/Switch A Circuit Intermittent |
| P0125 | Insufficient Coolant Temperature for Closed Loop Fuel Control |
Fixing a Throttle Position Sensor Circuit Fault for Good
A P0120 code means the PCM cannot trust the throttle position signal. Connectors, wiring and worn sensors cause most cases.
- Watch the TPS reading sweep smoothly in live data.
- Test the 5-volt reference, ground and signal.
- Check for a shared reference short if several codes appear.
- Replace the sensor or throttle body, then do a relearn if needed.
Frequently Asked Questions
How Do the Two Throttle Position Sensors Differ?
Drive-by-wire throttle bodies use two position sensors for safety. Sensor A and sensor B each report throttle angle, and the PCM cross-checks them. If one fails or they disagree, the PCM can limit power to prevent unintended acceleration. Older cable throttles usually have only sensor A.
Can a Dirty Throttle Body Set This Code?
Carbon buildup rarely sets a circuit code, since the wiring still works. It more often causes idle problems or a range and performance code. Cleaning the throttle body is still worth doing if it is dirty, but test the sensor circuit first when you see this code.
Do I Need to Relearn the Throttle After a Repair?
Many drive-by-wire engines need an idle or throttle relearn after a new throttle body, a cleaning or a battery disconnect. Some relearn on their own after a set key-on and idle sequence. Others need a scan tool. Your service manual lists the exact procedure.
Can a Bad Gas Pedal Sensor Cause the P0120 Code?
The accelerator pedal position sensor is separate from the throttle position sensor and sets its own codes. However, both often share the PCM’s 5-volt references. A shorted pedal sensor can pull a shared reference down and trigger throttle codes too. Check all stored codes before replacing parts.