P0120 – Throttle position (TP) sensor A/accelerator pedal position (APP) sensor / switch A -circuit malfunction

Mia

By Mia (Contact Me)
Last Updated 2016-06-03
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P0120 Throttle position (TP) sensor A/accelerator pedal position (APP) sensor A -circuit malfunction Wiring, TP/APP sensor, ECM

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What Does Code P0120 Mean?

The throttle position sensor (TPS) is located on the throttle body. It is a type of variable resistor which changes its resistance value according to throttle position. As the throttle is opened, the voltage reading goes up. This information is used by the PCM to determine output controls such as fuel injection and idle air control.

Most throttle position sensors are simple and easy to troubleshoot. There are three wires attached to the TPS: power, ground and signal. Power, or reference voltage, goes into the TPS. The sensor then sends a return signal voltage to the PCM. The return voltage depends on throttle positon. Typically, the signal value at idle will be about .45 volts. This voltage reading increases as the throttle is opened and will read about 4.5 volts at wide open throttle (WOT).

Many late model vehicles don’t use a throttle wire. Instead, they use what is referred to as “drive by wire”. In these systems, a motor is used to operate the throttle using inputs from position sensors on the accelerator pedal. When the gas pedal is depressed, the control module commands the throttle to open. Then, a pair of TPS sensors relay the throttle positon to the control module so it knows everything is working properly.

throttle-position-sensor

A typical throttle position sensor

(Courtesy: autozone.com)

Note: “Circuit Malfunction” indicates that there is a malfunction in the control circuit, as opposed to a fault in a sensor or other component. With “Circuit Malfunction” codes, replacement of sensors and components in the affected circuit will almost never resolve the problem, since as the code suggests, the trouble is in the circuit. This distinction between “circuit” and “sensor/component” is a great help to anyone trying to diagnose a circuit malfunction code, since it narrows the list of possible causes down considerably.

The causes of “Circuit Malfunction” codes are much the same as those for “Open Circuits” i.e., broken wiring, poor connections across electrical connectors or previously repaired wiring, loss of ground that prevents current flow, blown fuses, defective relays, faulty switches, or any of a host of other issues and problems that prevents a flow of current through wiring. Issues like high/low/intermittent voltages can set a “Circuit Malfunction” code on some applications. Moreover, “Circuit Malfunction” codes could also indicate a problem with negative current control / flow, as well as issues with failed or failing PCM’s (Powertrain Control Modules), although control module failure is a rare event.

What are the common causes of code P0120 ?

To sum things up, the common causes for code P0120 are as follows:

  • TPS sensor is faulty
  • TPS circuit is open or shorted
  • The PCM is faulty

What are the symptoms of code P0120 ?

Symptoms consistent with a faulty throttle position sensor include: hesitation or stumbling upon acceleration, unstable idle, and an illuminated check engine light.  In some cases, a no-start condition may even result.

How do you troubleshoot code P0120 ?

  • Visually inspect the sensor and its wiring

Start with a visual inspection of the sensor and its wiring. Make sure the connector is plugged in and free of corrosion; and check for broken wires.

  • Check the throttle body assembly

Check the throttle body assembly for binding and signs of contaminant buildup. If the throttle body is visibly dirty, remove it and clean it with throttle body cleaner. This is readily available at any auto parts store. Once you have cleaned the throttle body, re-install it and clear the codes. If they don’t come back, your problem is solved.

  • Test the sensor

If the visual inspection doesn’t reveal anything obvious, it’s time to move on to testing the sensor. This can easily be done using a digital multimeter set to the volts setting. Start by consulting the repair information for your vehicle to determine which wire is the signal wire. Leave the connector attached and insert one back probe test lead into the signal wire pin of the connector, and attach the other to ground. Open the throttle slowly; you should see the voltage increase smoothly as the throttle is opened. Consult the repair information for your vehicle to determine what the exact TPS readings should be. Typically, the sensor should read around 4.5 volts at wide open throttle and .45 volts with the throttle at the idle position.
back-probe

Back probing a connector

(Courtesy: enduringautomotive.com)

You can also test the TPS with a scan tool. Connect the scan tool to the diagnostic port, and scroll through the PID list until you find the TPS voltage or position PID. Slowly depress the gas pedal; you should see the TPS PID value increase. No change indicates there is no input from the sensor. More than 5% at idle or less than 90% at WOT indicates a potential problem with the sensor as well.

test-circuit

Testing the throttle positon sensor and its circuit

(Courtesy: aa1car.com)

Unfortunately, most scan tools and multimeters do not have a quick enough refresh rate to catch a momentary glitch in TPS operation. That’s why the best way to test the sensor is with an oscilloscope. With the scope, you can view the voltage signal as a wave form. This enables you to spot any fleeting problems.

tps-waveform

Typical TPS waveform pattern

(Courtesy: aa1car.com)

  • Test the TPS circuit wiring

If the TPS checks out OK, but you still have P0120 code illuminated, you’ll need to check the sensor circuit. First, you want to disconnect the sensor connector and test for power and ground. You can do this using a digital multimeter.

Start by consulting the repair information for your vehicle to determine which pin on the connector is power and which is ground. Next, set your multimeter to the volts setting. Touching the black multimeter lead to ground and the other to the power feed on the connector, you should see a reading that’s close to battery voltage. If not, you have a problem with the sensor power supply. You’ll need to consult the power side of the wiring diagram to find where the circuit fault lies.

To test the ground side of the circuit, connect the red multimeter lead to the battery positive terminal and the black lead to ground. Once again, you should see a reading of about 12 volts. If not, you’ll need to consult the ground side of the wiring diagram to find where the circuit fault lies.

If everything is OK up to this point, you’ll want to check the TPS signal wiring to the PCM. Using a digital multimeter set to the ohms, connect one side of the meter to the harness side of the TPS connector and the other to the TPS input pin on the PCM. This tests for continuity between the sensor and PCM. If your meter reads OL, there is an open somewhere in the circuit that must be repaired. If not, the PCM is likely the problem and should be replaced. However, PCMs rarely go bad, so make sure you double check all of your work before condemning the PCM.

Codes Related to P0120

  • DTC P0120: Throttle/Pedal Position Sensor/Switch A Circuit
  • DTC P0121: Throttle/Pedal Position Sensor/Switch A Range
  • DTC P0122: Throttle/Pedal Position Sensor/Switch A Circuit High
  • DTC P0123: Throttle/Pedal Position Sensor/Switch A Circuit Low
  • DTC P0124: Throttle/Pedal Position Sensor/Switch A Circuit Intermittent
  • DTC P0220: Throttle/Pedal Position Sensor/Switch ‘B’ Circuit
  • DTC P0221: Throttle/Pedal Position Sensor/Switch ‘B’ Circuit Range/Performance Problem
  • DTC P0222: Throttle/Pedal Position Sensor/Switch ‘B’ Circuit Low Input
  • DTC: P0223: Throttle/Pedal Position Sensor/Switch ‘B’ Circuit High Input
  • DTC: P0224: Throttle/Pedal Position Sensor/Switch ‘B’ Circuit Intermittent
  • DTC P0225: Throttle/Pedal Position Sensor/Switch ‘C’ Circuit
  • DTC P0226: Throttle/Pedal Position Sensor/Switch ‘C’ Circuit Range/Performance Problem
  • DTC P0227: Throttle/Pedal Position Sensor/Switch ‘C’ Circuit Low Input
  • DTC P0228: Throttle/Pedal Position Sensor/Switch ‘C’ Circuit High Input
  • DTC P0229: Throttle/Pedal Position Sensor/Switch ‘C’ Circuit Intermittent

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