Description of fault code P0117,
OBD2 Error Codes

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

26.05.2022 /

OBD-II Trouble Code - P0121 Technical Description

P0121 - Throttle Position Sensor/Switch A Circuit Range/Performance Problem.

DTC P0121 occurs when the engine control module (ECU, ECM or PCM) detects a faulty throttle position sensor (TPS - Throttle Position Sensor), also called a potentiometer, that is sending incorrect values ​​according to regulations.

What does trouble code P0121 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles. Although general, specific repair steps may differ depending on the brand / model.

The throttle position sensor is a potentiometer that measures the amount of throttle opening. As the throttle is opened, the reading (measured in volts) increases.

The powertrain control module (PCM) supplies a 5V reference signal to the Throttle Position Sensor (TPS) and usually also to ground. General measurement: idle = 5V; full throttle = 4.5 volts. If the PCM detects that the throttle angle is greater or less than it should be for a certain RPM, it will set this code.

Possible symptoms

Symptoms of a P0121 trouble code may include:

  • Malfunction Indicator Lamp (MIL) illuminated (Check Engine Light or Engine Service Soon)
  • Intermittent stumbling when accelerating or decelerating
  • Blowing black smoke when accelerating
  • Does not start
  • Turn on the corresponding engine warning light.
  • General engine malfunction, which can lead to a misfire.
  • Problems with accelerating maneuvers.
  • Problems with starting the engine.
  • Increased fuel consumption.

However, these symptoms may also appear in combination with other error codes.

Causes of the P0121 code

The throttle position sensor performs the task of monitoring and determining the opening angle of this damper. The recorded information is then sent to the engine control unit, which uses it to calculate the amount of fuel needed to be injected into the circuit to achieve perfect combustion. If the engine control module detects an erratic throttle position due to a faulty position sensor, DTC P0121 will automatically set.

The P0121 code may mean that one or more of the following events have occurred:

  • Throttle position sensor malfunction.
  • Wiring fault due to bare wire or short circuit.
  • Throttle position sensor wiring problem.
  • Presence of moisture or external intrusions that affect the operation of the electrical system.
  • Faulty connectors.
  • Malfunction of the engine control module, sending incorrect codes.
  • The TPS has an intermittent open circuit or an internal short circuit.
  • The harness is rubbing, causing an open or short circuit in the wiring.
  • Bad connection in TPS
  • Bad PCM (less likely)
  • Water or corrosion in connector or sensor

Possible solutions

1. If you have access to a scan tool, see what the idle and wide open throttle (WOT) readings for the TPS are. Make sure they are close to the specifications mentioned above. If not, replace the TPS and recheck.

2. Check for intermittent open or short circuit in the TPS signal. You cannot use a scan tool for this. You will need an oscillator. This is because scanning tools take samples of many different readings on just one or two lines of data and may miss intermittent dropouts. Connect an oscilloscope and observe the signal. It should rise and fall smoothly, without dropping out or protruding.

3. If no problem is found, perform a wiggle test. Do this by wiggling the connector and harness while observing the pattern. Drops out? If so, replace the TPS and recheck.

4. If you don't have a TPS signal, check for a 5V reference on the connector. If present, test the ground circuit for an open or short circuit.

5. Make sure the signal circuit is not 12V. It should never have battery voltage. If so, trace the circuit for a short to voltage and repair.

6. Look for water in the connector and replace the TPS if necessary.

Other TPS Sensor and Circuit DTCs: P0120, P0122, P0123, P0124

Repair Tips

After the vehicle is taken to the workshop, the mechanic will usually perform the following steps to properly diagnose the problem:

  • Scan for error codes with an appropriate OBC-II scanner. Once this is done and after the codes have been reset, we will continue to test drive on the road to see if the codes reappear.
  • Checking the throttle position sensor.
  • Inspection of cable system components.
  • Throttle valve inspection.
  • Measuring the resistance of the sensor with a suitable instrument.
  • Inspection of connectors.

A quick replacement of the throttle sensor is not recommended, as the cause of the P0121 DTC may lie in something else, such as a short circuit or bad connectors.

Generally, the repair that most often cleans up this code is as follows:

  • Repair or replace the throttle position sensor.
  • Repair or replacement of connectors.
  • Repair or replacement of faulty electrical wiring elements.

Driving with error code P0121 is not recommended, as this can seriously affect the stability of the car on the road. For this reason, you should get your car to the workshop as soon as possible. Given the complexity of the inspections being carried out, the DIY option in the home garage is unfortunately not feasible.

It is difficult to estimate the upcoming costs, since a lot depends on the results of the diagnostics carried out by the mechanic. Typically, the cost of repairing a throttle body in a workshop can exceed 300 euros.

P0121 Throttle Postition Sensor troubleshooting tips

Frequently Asked Questions (FAQ)

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NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.

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