Error Code P0120: Throttle Position Sensor/Switch (TPS) A Circuit Malfunction

Error Code P0120 is described as Throttle Position Sensor (TPS) or Switch TPS “A” Circuit Malfunction. Meaning, the PCM (powertrain control module, also referred to as ECM or engine control module) has determined that the output voltage of the TPS sensor circuit A has gone lower or higher than its expected range, or is chattering compared to its B counterpart.

It’s a generic trouble code, which means it applies to all vehicles made since 1996 to present. Obviously, different makes and models have different definition specifications and troubleshooting information. This code is related to P0121, P0122, P0123, and P0124.

Definition

The job of the TPS is to respond to the movement of the accelerator pedal. It’s basically a potentiometer which transforms the position of the throttle into an output voltage, emitting the signal of the voltage to the PCM. It can be found in the throttle body of Intake Manifold, while the Pedal Position Sensor/Switch is part of Accelerator Pedal assembly. These sensors provide the much needed input from the foot of the driver in terms of how much power the engine needs and how soon it is needed. The TPS rotates from its base resting position for a full acceleration, which sends an increasing signal to the PCM. This increase and decrease of voltage signal is then used by the PCM to manage different aspects of Emission System Components, such as Air Fuel Ratio, engine’s Spark Timing, etc.

Aside from that, the sensor also detects throttle valve’s opening and closing speed, and feeds the needed voltage signal to the PCM.

Error Code P0120 is detected when the circuit “A” malfunctions, or there is an excessively high or low voltage from the sensor sent by the TPS to the PCM.

Common Symptoms

Aside from illumination of the Check Engine light, this error code may also show some noticeable drivability issues such as hard starting and hesitation on acceleration. Other symptoms include:

  • Possible starts and stalls
  • Poor idle quality or misfiring idle at highway speed

Note that these symptoms are determined by the manufacturer. Different makes and models have different symptoms, as well as the exact cause of the problem. This error code puts the vehicle in failsafe mode, meaning it cuts the current to the throttle actuator, putting the throttle valve at around 6˚ opening. The vehicle can be driven very slowly, but with limited throttle response.

Possible Causes

Possible causes for P0120 includes:

  • Defective throttle or pedal position sensor or switch
  • Jammed or torn floor mats
  • Dirty throttle bore (or carbon filled)
  • Corroded or faulty TPS or switch
  • Corroded or faulty TPS wiring harness
  • Misrouted harness leading to chaffing
  • Corroded MAP (manifold pressure)
  • Bad PCM (rare)

How to Check

As with other error codes, it is important to record the freeze frame rate data and duplicate the conditions when the error code was set prior to a test drive. A qualified mechanic will use his data streaming scan tool and pay close attention to the engine load, RPM, throttle position, and road speed. When driving the vehicle, values will be compared to Intake Air Temperature sensor parameter ID. The Intake Air Temperature sensor voltage values should rise and fall depending on the changes in engine temperature. He will then compare the IAT readings to the readings from the Coolant Temperature sensor, which should move in tandem with each other. The temperature of engine coolant sensor will have a slightly higher range.

With key in, engine off, the IAT sensor connector must be checked. There should be 5V reference voltage and a good ground. Find your vehicle’s proper engine performance wiring diagram to determine the proper color and position of these connectors and wires.

How to Fix

As with other generic trouble codes, P0120 is pretty easy to fix; some of the most common solutions are:

  • Repair or replacement of TPS connector
  • Repair any damaged wiring
  • Replacement of TPS sensor

As said earlier, this code puts the PCM to failsafe mode, meaning it will cause various drive complaints, such as bucking or jerking, high idle, lack of acceleration, limited speed or even stalling until it’s fixed.

When repairing for this trouble code, it is important to conduct a thorough diagnosis to pinpoint the exact cause of the problem. Conduct a thorough visual test, and do not replace the TPS sensor unless you’re sure it’s what’s really causing the problem.

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