P0121 – Throttle/Pedal position sensor/switch “A” circuit

The P0121 code triggers when the “A” and “B” circuits of your vehicle’s throttle position sensor don’t match, which can be a severe problem, activating your car’s failsafe mode and limiting engine performance.

While the throttle position sensor failures are the most common cause of the P0121 trouble code, you don’t want to replace it. There may be issues with the wiring or more extensive electrical problems. Conducting a full diagnosis is essential to make sure the code doesn’t return.

In many cases, the P0121 trouble code can be quickly cleared in a home garage. Read on below to learn more about what causes it and how you can identify and eliminate the problem. 

P0121 code definition

P0121 code definition (generic): Throttle position sensor/switch A circuit range/performance problem

P0121 Dodge code definition: Throttle Pos. Exceed (Short Time)

P0121 Ford code definition: (TP) sensor circuit performance problem

P0121 Nissan code definition: (TP) sensor circuit performance problem

P0121 Toyota code definition: Throttle Pedal Position Sensor/Switch “A” Circuit Range/Performance

Here's where you can get a thorough understanding of the P0121 OBD2 code
If you want to learn about the P0121 OBD2 code, this is the right place

What does P0121 mean?

The sensor that monitors your throttle’s movement is the throttle position sensor (TPS). This sensor is a potentiometer, measuring the current that passes through to indicate the throttle position. As the throttle opens, the voltage goes up.

The TPS circuit has a designated output voltage for both the “A” and the “B” circuit. This voltage is monitored by the engine control module (ECM) to make sure the throttle position is staying where it should be. The ECM also sends a 5-volt reference signal to the sensor. Usually, the sensor will measure around .5 volts when idling and as much as 4.5 volts when at full throttle.

The “A” and “B” circuits of the TPS sensor should maintain a particular relationship with each other. When the P0121 OBD2 code triggers, the ECM has detected a TPS “A” circuit voltage that’s either too high or too low compared to the “B” circuit. 

P0121 is a generic powertrain code, so it applies to any vehicle equipped with OBD2. You may want to follow different steps for the diagnosis and repair depending on your vehicle manufacturer, however. Check for any technical service bulletins for this code for your vehicle. 

What are the symptoms of the P0121 code?

When the P0121 trouble code activates, the engine will go into failsafe mode, limiting the throttle’s response, which prevents your car from driving above a certain speed. In addition to the failsafe mode, you’ll likely notice some of the following symptoms:

  • Activation of the check engine light
  • Knocking sounds in the engine
  • Engine stalling or refusing to start
  • Vehicle accelerating slowly
  • Smoke in the exhaust when accelerating
  • Engine stumbling and jerking when changing speeds
  • Lower maximum speed
Failures of the throttle position sensor triggers the P0121 code
The P0121 code indicates a problem with the throttle position sensor

What are the causes of P0121?

  • Open or short in TPS
  • Open or short in the TPS wiring
  • Faulty or loose connections on the TPS
  • Corrosion or moisture in sensor connections
  • Failed or faulty ECM (rare)

How serious is the P0121 code?

The P0121 trouble code is very severe. Only the activation of failsafe mode will make your car difficult to drive. It may also prevent your car from starting or lead to stalling if it does run. You should stop driving your vehicle when this code activates and fix the problem immediately.

How to diagnose and fix the P0121 code

Tools you’ll need:

The trouble code P0121 can be diagnosed with an OBD2 scan tool
Use the OBD2 scan tool to diagnose the P0121 code


  1. Scan the TPS data using an OBD2 scan tool. Compare the readings for circuit “A” and circuit “B.” If there’s a variance, check your manual for manufacturer tests for the TPS.
  2. Scan your system to see if any other trouble codes have been triggered. You may see other codes related to the throttle position sensor (P0120-P0124). 
  3. Use the OBD2 scan tool to read the idle and wide-open throttle (WOT) readings. Compare them with the specifications in your vehicle’s manual. If they’re not, this indicates an issue with the TPS, wiring, or connectors.
  4. Visually inspect all the wiring and connections of the TPS system. Replace any wires that are damaged or corroded, and make sure all connections are secure. Also, inspect the TPS connections for corrosion or moisture damage. After your inspection, clear the codes and test drive your vehicle. If the code returns, continue with your diagnosis.
  5. Use an oscilloscope to check for an open or short in the signal sent by the TPS. The signal should go up and down smoothly. If you see sharp drops or spikes, the sensor should be replaced.
  6. Use a digital multimeter to check the reference voltage at the connector. It should read 5 volts. If it doesn’t, check the ground circuit. It may have an open or short. If the signal circuit reads 12 volts, trace the circuit to find the source of the short and repair it.
  7. If the code still doesn’t clear, take your vehicle to a mechanic for further diagnosis. In rare instances, the P0121 triggers because of a more serious electrical problem. A professional will be able to determine if this is the case.

Common mistakes to avoid while diagnosing the P0121 code

Many mechanics immediately replace the TPS before visually inspecting the system for corroded wires, loose connections, and other simple fixes. Make sure you conduct a full, thorough diagnosis before replacing any components. If the wiring is the problem, replacing the sensor won’t fix it. 

How to fix the P0121 code

Tips to avoid P0121 in the future

If the TPS wires aren’t installed carefully, they can rub against the wiring harness., which leads to damaged wires, one of the primary causes of the P0121 trouble code. Ensure the wires in your system aren’t touching anything that could damage them to prevent shorts and other electrical issues. Moisture also damages electrical components, like sensors. If you see signs of moisture on the TPS or its connections, figure out how it’s getting into your engine. Identifying and fixing the source of the leak will prevent future damage to the TPS.

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