P0121 – Throttle/Pedal Position Sensor/Switch “A” Circuit

If you want to learn about the P0121 OBD2 code, this is the right place

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

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

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

The P0121 code indicates a problem with the throttle position sensor

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

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 what position the throttle is in. 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 certain relationship to 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. This limits the response of the throttle, 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

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. Simply 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:

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 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 won’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 they visually inspect 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. This leads to damaged wires, one of the main causes of the P0121 trouble code. Make sure 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.

Read more: P0335 Code: Meaning, Symptoms, Causes, Diagnostics, and Fixes