P0120 is a common OBD-II trouble code that points to a problem with how your engine computer reads throttle position. When this code sets, your car may lose power, hesitate, or even go into “limp” mode. While it can feel like a major failure, P0120 is usually straightforward to diagnose with a scan tool and a multimeter. In this guide, you’ll learn what P0120 actually means, the most likely causes, what symptoms to watch for, and the typical repairs and costs so you can decide what to do next.
What Does P0120 Mean?
P0120 stands for “Throttle/Pedal Position Sensor/Switch A Circuit Malfunction.” In simple terms, the engine control unit (ECU) is not happy with the electrical signal coming from the main throttle position sensor (TPS) or accelerator pedal position sensor, depending on your setup.
The TPS tells the ECU how far the throttle is open so it can control fuel, ignition timing, and, on drive-by-wire systems, the electronic throttle body. When the signal is missing, out of range, or inconsistent, the ECU sets P0120 and may limit throttle response to protect the engine and transmission.
Quick Reference
- Code: P0120
- Definition: Throttle/Pedal Position Sensor/Switch A Circuit Malfunction
- Main area: Throttle position sensor or accelerator pedal sensor circuit
- Severity: Moderate to high – can cause drivability and safety issues
- Typical fix: Sensor replacement, wiring repair, throttle body service or replacement
Real-World Example / Field Notes
In the shop, I see P0120 most often on vehicles with high mileage or after a DIY throttle body cleaning. One example: a mid-2000s sedan came in with poor acceleration and a flashing check engine light. The owner had just cleaned the throttle body and unplugged the TPS. The connector wasn’t fully seated and one terminal was slightly bent. The ECU saw an erratic TPS signal and set P0120. Straightening the pin, cleaning the connector, clearing the code, and performing a throttle relearn fixed the issue without replacing any parts.
Symptoms of P0120
- Check engine light on: P0120 usually sets the MIL and may store additional throttle-related codes.
- Poor acceleration: You may notice sluggish response, especially when trying to pass or merge.
- Limp mode: Some vehicles limit throttle opening and RPM to protect the engine and transmission.
- Rough idle: Unstable or high idle speed as the ECU struggles to control airflow.
- Stalling: Engine may stall when coming to a stop or when you tap the throttle.
- Surging or hesitation: Jerky throttle response or intermittent hesitation on light acceleration.
- Poor fuel economy: Incorrect throttle data can lead to rich or lean mixtures and wasted fuel.
Common Causes of P0120
Most Common Causes
- Failed throttle position sensor (TPS): Internal wear, heat, and vibration can cause dead spots or erratic voltage output.
- Faulty accelerator pedal position sensor: On drive-by-wire systems, the pedal sensor often shares the same “A” circuit monitored by the ECU.
- Wiring issues in the TPS circuit: Broken wires, poor splices, or chafed insulation causing shorts to ground or power.
- Corroded or loose connectors: Moisture, oil, or dirt in the TPS or pedal connector causing high resistance or intermittent contact.
- Dirty or sticking electronic throttle body: Excessive carbon buildup causing the throttle plate to stick and confuse the position feedback.
Less Common Causes
- Incorrect throttle body cleaning: Over-rotating the throttle plate by hand or using harsh chemicals damaging the TPS.
- Bad ECU (PCM) driver: Internal fault in the control unit affecting the 5V reference, ground, or signal processing.
- Low system voltage: Weak battery, poor grounds, or alternator issues causing unstable sensor readings.
- Aftermarket throttle body or pedal assembly: Non-OEM parts with slightly different calibration confusing the ECU.
- Water intrusion: Flood damage or deep water crossings affecting connectors and harnesses near the throttle body.
Diagnosis: Step-by-Step Guide
To properly diagnose P0120, you’ll want at least a basic OBD-II scan tool, a digital multimeter, and ideally a wiring diagram for your specific vehicle. A scan tool that can read live data and Mode $06 information makes this much easier. If you’re not comfortable working around a running engine and electrical circuits, consider having a professional handle these steps.
- Confirm the code and check for companions. Scan the vehicle and note all stored and pending codes. Look for related throttle or pedal codes like P0121–P0124 or P0220–P0229. Multiple codes can point to a shared power or ground issue.
- Check live TPS and pedal data. In the scan tool data stream, watch TPS and accelerator pedal position while slowly pressing and releasing the pedal. The values should increase smoothly from about 0–20% at idle to near 100% at wide open. Any jumps, dropouts, or flat spots indicate a sensor or wiring issue.
- Perform a visual inspection. With the key off, inspect the throttle body, TPS, and pedal sensor connectors. Look for broken locks, corrosion, oil contamination, or damaged wiring looms. Gently tug on wires to check for loose or broken conductors.
- Verify 5V reference and ground. Using a multimeter back-probed at the TPS connector, check for a stable 5V reference and a solid ground with the key on, engine off. If either is missing, trace back to the ECU and shared sensors that use the same 5V line.
- Measure TPS signal voltage. With the connector plugged in and the key on, back-probe the signal wire. Slowly open the throttle (or have a helper press the pedal). Voltage should rise smoothly from around 0.5V at closed throttle to about 4.5V at wide open. Any sudden jumps or dead spots usually mean a bad sensor.
- Wiggle test the harness. While watching the live data or multimeter reading, gently wiggle the harness and connectors. If the signal cuts out or spikes, you likely have a broken wire or poor connection.
- Inspect and clean the throttle body. If the throttle plate is heavily carboned up, clean it with proper throttle body cleaner and a lint-free cloth. Do not force the plate open against its stop on electronic units. After cleaning, some vehicles require a throttle relearn procedure with a scan tool or specific key-on/key-off sequence.
- Check Mode $06 and freeze frame. Use Mode $06 (if available) to see if the TPS monitor is marginal or failing intermittently. Freeze frame data tells you what throttle angle, RPM, and speed were present when P0120 set, which can help you recreate the conditions.
- Test continuity to the ECU. If power, ground, and sensor seem okay, unplug the ECU and TPS, then check continuity and resistance of each circuit wire. Any high resistance or open circuit needs repair.
- Substitute a known-good sensor (if possible). On some platforms, swapping in a known-good TPS or pedal sensor is the quickest way to confirm a bad part after basic checks.
Pro tip: Always clear the code and perform a proper road test under similar conditions to when the code first set. A quick spin around the block at light throttle may not be enough to prove the fix.
Possible Fixes & Repair Costs
Most P0120 repairs involve replacing a faulty sensor or repairing wiring. A typical throttle position sensor or accelerator pedal sensor replacement at an independent shop runs about $150–$450 parts and labor, depending on vehicle and part quality. Electronic throttle body replacement can range from $350–$900. Wiring repairs vary widely, from $100 for a simple connector fix to $400+ for harness tracing and repair. Labor rates, OEM vs aftermarket parts, and how hard the components are to access all affect the final bill.
- Replace throttle position sensor (TPS) and clear codes.
- Replace accelerator pedal position sensor or pedal assembly.
- Repair or replace damaged wiring and connectors in the TPS circuit.
- Clean or replace the electronic throttle body and perform throttle relearn.
- Update ECU software (reflash) if a known calibration issue exists.
- In rare cases, replace a faulty ECU after all other causes are ruled out.
Can I Still Drive With P0120?
You can often start and move the vehicle with P0120 present, but it’s not something you should ignore or drive with for long. Many cars will go into limp mode, limiting power and speed, which can be dangerous when merging or passing. Unpredictable throttle response, stalling, or sudden loss of power are all real possibilities. If the car drives normally, you may be okay to carefully drive to a shop. If it runs poorly or stalls, have it towed to avoid getting stranded or causing an accident.
What Happens If You Ignore P0120?
Ignoring P0120 can lead to chronic drivability issues, poor fuel economy, and potential safety risks from sudden loss of power or stalling. Over time, running with incorrect throttle data can stress the engine, catalytic converter, and transmission. You also lose the ability to detect new problems because the check engine light is already on. Fixing P0120 sooner rather than later usually saves money and headaches.
Related Codes
- P0129 – Barometric Pressure Too Low
- P0127 – Intake Air Temperature Too High
- P0126 – Insufficient Coolant Temperature for Stable Operation
- P0125 – Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0124 – Throttle/Pedal Position Sensor/Switch “A” Circuit Intermittent
- P0123 – Throttle/Pedal Position Sensor/Switch “A” Circuit High
- P0122 – Throttle/Pedal Position Sensor/Switch “A” Circuit Low
- P0121 – Throttle/Pedal Position Sensor/Switch “A” Circuit Range/Performance
- P0119 – Engine Coolant Temperature Circuit Intermittent
- P0116 – Engine Coolant Temperature Circuit Range/Performance
Key Takeaways
- P0120 points to a problem with the main throttle or pedal position sensor circuit, not just a random electrical glitch.
- Common causes include a bad TPS, worn pedal sensor, wiring issues, or a dirty/sticking electronic throttle body.
- Symptoms range from a simple check engine light to limp mode, stalling, and poor acceleration.
- Basic diagnosis with a scan tool and multimeter can quickly separate sensor failure from wiring or ECU problems.
- Typical repairs cost between $150 and $900 depending on which parts are replaced and labor rates in your area.
- Driving long-term with P0120 is not recommended due to safety and potential engine and emissions damage.
Vehicles Commonly Affected by P0120
P0120 shows up across many makes because every modern vehicle uses some form of throttle or pedal position sensor. In the bay, I see it frequently on:
- Toyota and Lexus: Camry, Corolla, RAV4, Tacoma, and similar models with electronic throttle bodies.
- GM vehicles: Chevrolet Silverado, Tahoe, Malibu, GMC Sierra, and Buick sedans with drive-by-wire systems.
- Ford and Lincoln: F-150, Escape, Fusion, Explorer, and related SUVs and trucks.
- Chrysler, Dodge, Jeep, Ram: Minivans, Chargers, Grand Cherokee, and Ram trucks with pedal and throttle issues.
- Hyundai and Kia: Elantra, Sonata, Santa Fe, and similar crossovers.
- Nissan and Infiniti: Altima, Maxima, Rogue, and various VQ-engine models.
The exact failure pattern varies by manufacturer, but the diagnostic approach is similar on most OBD-II vehicles.
FAQ
Can I clear P0120 and keep driving if the car feels fine?
You can clear the code, but if the underlying problem is still there, P0120 will usually come back. If the car drives normally after clearing, you can carefully monitor it, but plan on at least inspecting the throttle body, wiring, and connectors soon.
Is P0120 always caused by a bad throttle position sensor?
No. A failed TPS is common, but wiring problems, loose connectors, pedal sensor issues, or a dirty electronic throttle body can all trigger P0120. Testing voltage, ground, and signal before replacing parts saves money and avoids guesswork.
How do I know if it’s the pedal sensor or the throttle body?
A scan tool that shows both pedal and throttle position is the best way. If pedal data looks smooth but throttle position is erratic, suspect the throttle body or TPS. If both are erratic or don’t agree, you may have a wiring or 5V reference issue, or a bad pedal sensor.
Can a weak battery or alternator cause P0120?
<pYes, in some cases. Low system voltage or voltage spikes can disturb sensor readings and cause throttle-related codes. If you also have dim lights, slow cranking, or charging system codes, test the battery and alternator as part of your diagnosis.
Do I need a throttle relearn after fixing P0120?
<pMany modern vehicles require a throttle or idle relearn after replacing the throttle body, TPS, or cleaning heavy carbon buildup. Some relearns happen automatically after a few drive cycles; others need a scan tool or a specific key-on sequence. Check service info for your vehicle so the repair holds.