P0120 Code (Symptoms, Causes, and How to Fix)

Is your check engine light on and the subsequent scan shows a code P0120? Don’t panic just yet. While some trouble codes indicate major engine problems, P0120 usually points to a relatively straightforward issue with your throttle position sensor that won’t empty your wallet to fix.

Keep reading to learn what a P0120 code means, its warning signs, most likely causes, and how you can diagnose and potentially fix this problem in your driveway with basic tools.

Table of Contents

What Does Code P0120 Mean?

OBD-II Trouble Code P0120 Description

Throttle/Pedal Position Sensor/Switch "A" Circuit Malfunction

P0120 is a diagnostic trouble code that points to a problem with your vehicle’s throttle position sensor (TPS) circuit. In simple terms, your car’s computer has detected an issue with the signal coming from the main throttle position sensor.

The throttle position sensor works like a messenger between your gas pedal and the engine. When you press the pedal, this sensor sends a voltage signal to your car’s computer (ECM/PCM), which then adjusts how much fuel gets into the engine. The sensor operates as a variable resistor that changes a 5-volt reference signal based on how far you press the gas pedal.

When the P0120 code appears, it means the computer is receiving incorrect, inconsistent, or no signal at all from the throttle position sensor.

Unlike the related codes P0121 (range/performance issue), P0122 (low input), or P0123 (high input), P0120 indicates a more general circuit malfunction that could be caused by various electrical problems.

Symptoms of Code P0120

When your vehicle registers a P0120 code, you’ll likely notice several symptoms that can affect how your car drives. These symptoms can range from mildly annoying to potentially dangerous:

Not every vehicle will show all these symptoms. The severity often depends on how your specific car model is programmed to respond to this fault and how badly the sensor or circuit is damaged.

Causes of Code P0120

The P0120 code can be triggered by several different issues. Knowing these possible causes can help you focus your troubleshooting efforts:

Damaged or failing throttle position sensor
Loose or corroded electrical connections
Damaged wiring in the TPS circuit
Water or fluid damage to the sensor or connector
Carbon buildup on the throttle body
Improper throttle position sensor adjustment

In many modern vehicles with electronic throttle control (drive-by-wire systems), this issue can be particularly complex since there’s no mechanical connection between your gas pedal and the engine.

Is Code P0120 Serious?

Yes, P0120 should be considered a serious code that needs prompt attention. The throttle position sensor directly affects how your engine responds to your commands as a driver.

With a malfunctioning TPS, your vehicle might unexpectedly stall in traffic, fail to accelerate when needed, or surge forward unpredictably. These behaviors create genuine safety concerns, particularly when merging onto highways or crossing busy intersections.

Additionally, continued driving with this issue could potentially lead to further damage to your vehicle’s engine management system. The computer might try to compensate for bad sensor data by adjusting fuel mixture inappropriately, which could affect other components over time.

If you notice symptoms of a P0120 code, it’s best to have your vehicle checked as soon as possible. If you’re uncomfortable making these repairs yourself, schedule an appointment with a trusted mechanic.

How to Fix Code P0120

Follow these steps to diagnose and fix the P0120 code. Before starting, gather basic tools including a multimeter, screwdrivers, and throttle body cleaner. Always refer to your vehicle’s service manual for specific procedures and specifications.

#1 – Check for Additional DTCs

First, scan your vehicle for any other trouble codes that might be present. Related throttle position sensor codes or other electrical codes can provide helpful clues about the bigger picture. Fix any additional unrelated codes before proceeding.

#2 – Inspect the TPS and Wiring

Start with a visual inspection of the throttle position sensor and its wiring harness. Look for:

Loose or damaged connectors
Frayed or broken wires
Signs of water damage or corrosion
Physical damage to the sensor itself

Disconnect the battery before handling any electrical components. Clean any corroded connections you find using electrical contact cleaner and a small wire brush.

#3 – Clean the Throttle Body

Carbon buildup on the throttle body can interfere with proper throttle plate movement and sensor readings. Clean the throttle body using an appropriate cleaner and a soft cloth.

For mechanical throttle bodies, you can usually clean them while installed. Electronic throttle bodies may require removal for proper cleaning. Be careful not to spray cleaner directly onto electrical components.

Some vehicles with electronic throttle bodies require a throttle position relearn procedure after cleaning. Check your service manual for details.

#4 – Test the TPS Circuit

Using a multimeter, check the throttle position sensor circuit:

With the ignition on but engine off, verify the sensor receives 5 volts on the reference wire.
Check that the ground wire has continuity to ground.
Slowly operate the throttle while monitoring the signal voltage. It should change smoothly from about 0.5V (closed throttle) to 4.5V (wide open).
Any dead spots or jumps in the voltage reading indicate a faulty sensor.

#5 – Replace the TPS if Necessary

If testing confirms the throttle position sensor is faulty, replace it with a quality part. In many vehicles, the TPS is integrated with the throttle body assembly and must be replaced as a unit.

After replacement, some vehicles require a computer relearn procedure using a scan tool. This helps the ECM/PCM adapt to the new sensor’s parameters.

#6 – Final Verification

After repairs, clear the trouble codes and test drive the vehicle under various conditions. Monitor for proper throttle response and make sure the check engine light doesn’t return.

If symptoms persist or the code returns, there might be an issue with the engine control module itself, though this is relatively rare.

Remember that modern throttle systems are complex, integrating multiple sensors and safety features. If you’re not comfortable with electrical diagnostics, this job might be best left to a professional mechanic.

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Mark Stevens

Owner at Car Treatments

Mark is a veteran auto technician who has honed his skills with over 19 years of service working at various independent shops. While Fords and Mazdas are his specialty, he enjoys working on practically any vehicle (except a Nissan 300ZX).

He’s known for his attention to detail and problem-solving skills and enjoys coming up with out-of-the box solutions when the situation requires it.

When he's not troubleshooting customers' cars, you'll most likely find him working on his current project ('96 Bronco) or cooking Italian food for his family.