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P0120 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

P0120 is an OBD-II Code that refers to Throttle Pedal Position Sensor/Switch (TPS) “A” Circuit Malfunction

Meaning

A P0120 code is caused by an ECM (Engine Control Module) receiving a signal that the Throttle Pedal Position Sensor/Switch is malfunctioning out of the acceptable range. When this happens, the ECM will set a P0120 code.

Causes

Common causes for this code include:

  • Chafing is a common problem caused by a misrouted harness
  • The throttle return spring is sticking
  • Bad TPS 
  • Corrosion in the TPS or MAP connector
  • Bad PCM

Symptoms

The symptoms of a P0120 code are:

  • MIL (Malfunction Indicator Light) is on 
  • Loss of performance 
  • Engine may die at idle and then restart itself 
  • Decelerates while driving and then speeds back up after a few seconds 
  • It’s possible that the engine will start and stall

Diagnosis

To diagnose a P0120 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure conditions
  2. Check for evidence of chafing and wiring problems 
  3. Clean the Throttle Body and Throttle Position Sensor/Switch 
  4. Test the Throttle Position Sensor/Switch using a Digital Multimeter (DMM) set to Ohms or Continuity 
  5. Check the MAP sensor and wiring for chafing or broken wires 
  6. Test drive and if all of these steps fail, replacing the PCM (Powertrain Control Module)

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P0120 code:

  • Not checking for chafing or wiring problems 
  • Replace parts without testing them 
  • Replacing the PCM when all signs lead to a bad Throttle Position Sensor/Switch or wiring harness 
  • Before and after repair or replacements, compare TPS sensor A and B to be within specification

How serious is this?

A P0120 code is not typically serious unless it is accompanied by other codes. If the issue is due to corroded wiring, chafing or faulty TPS and left fort too long, then there may be costly repairs.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Replacing a corroded wiring harness
  • Replacing a damaged throttle return spring
  • Reseating the TPS or MAP sensor connector 
  • Replacing a bad PCM (Powertrain Control Module)

Related codes

A P0120 is related to and may be accompanied by the following codes:

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

P0122 – Throttle/Pedal Position Sensor/Switch A Circuit Low Input

P0123 – Throttle/Pedal Position Sensor/Switch A Circuit High Input

Conclusion

In conclusion, the P0120 code is a generic trouble code that means the computer has detected an issue with the Throttle Pedal Position Sensor/Switch (TPS) “A” Circuit Malfunction and needs to be diagnosed and repaired. The potential causes for this code are chafing, wiring problems, a faulty TPS or MAP sensor connector or a bad PCM (Powertrain Control Module). Replacing the PCM is not recommended until all other possibilities have been eliminated as the cause.