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Code P0121: Throttle Position Sensor/Switch A Circuit Range/Performance Problem

When your OBD-II sensor shows trouble code P0121, there may be an electrical issue with your throttle position sensor. Learn how to diagnose this issue and find the correct repair solution in order to answer, how do I fix trouble code P0121?

What Does This Error Code Mean?

Trouble code P0121 is signaled when your powertrain control module receives an incorrect signal from your throttle position sensor, or TPS. Your TPS is connected to a reference wire with five volts and typically a ground wire. It communicates the position of your throttle with your powertrain control module, which uses your RPMs to check the signal.

While the specific signals may vary depending on your particular vehicle, for most throttle position sensors the idle voltage is 0.5 volts. This is the minimum signal your powertrain control module expects. The maximum expected signal is 4.5 volts at full throttle. Any signal outside this range, or any signal that doesn’t correspond with a correct RPM level, is identified as a fault in the TPS circuit.

Symptoms

So, what does the code P0121 mean? In most cases, it’s a sign that your TPS, ground wire or wire harness is damaged. Here are some symptoms you can expect from this trouble code:

  • Failure to start your engine
  • Occasional stumble when accelerating or decelerating
  • Exhaust emitting black smoke during acceleration
  • Malfunction indicator lamp illumination

Even if you aren’t experiencing these symptoms yet, it’s important to find the cause and restore your TPS reading. Without it, you may experience other performance issues unexpectedly. Repairing this integral sensor should restore your engine’s performance and prevent stumbling.

Potential Causes

Your TPS helps regulate your throttle and communicates this essential information to your powertrain control module. These symptoms are likely caused by any number of failures throughout the TPS electrical system. Careful inspection and prompt repairs should restore a clear and accurate reading.

The most common is a damaged wire harness. Over time, wiring can be damaged from excessive heat, moisture or strain. The length of wiring or connectors could have major or minor damage that results in an improper TPS reading.

Another common cause is an internal short in the TPS. A damaged sensor will either give faulty readings or will cease to send signals to your powertrain control module. Finally, although rare, your control module itself could be damaged.

Repair Solutions

Solve error code P0121 by going step-by-step through each potential causes and making any necessary repairs. You’ll need a voltmeter and scan tool for best results and to speed up the process.

Starting with your TPS sensor, test the readings with your scan tool. Look for accurate wide open throttle and idle readings. Compare your scan tool results with your service manual to determine whether your sensor needs to be replaced. Any inaccurate readings are indications that there’s internal wiring issues in your sensor.

Next, check the connectors of your wire harness. Inspect the entire length of wire leading from your powertrain control module to your TPS. Pay particular attention to connection points. Any loose or corroded connections need to be repaired or replaced. The wire harness is susceptible to water damage, so if you see any signs of moisture you may need to replace the entire harness.


A TPS sensor circuit fault is the most likely cause of your OBD-II scanner error code P0121 problem. This sensor can also be responsible for error codes P0120, P0122, P0123 and P0124. After you’ve made a repair, you may need to clear the code and test drive your vehicle. If P0121 or any other related trouble code appears again, revisit these causes and look for another fault in the TPS circuit.

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