Error Code P0123: Throttle Position Sensor/Switch A Circuit High Input

Error Code P0123 is described as Throttle Position Sensor (TPS)/Switch A Circuit High Input. Meaning, the PCM (powertrain control module, also referred as ECM or engine control module) has determined the TPS Circuit A output voltage range has gone higher than the sensor’s voltage specification.

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 the throttle valve’s opening and closing speed, and feeds the needed voltage signal to the PCM.

Error Code P0123 is triggered when the PCM reads that the TPS Switch A circuit has high output, or higher than the specified voltage range.

Common Symptoms

Error Code P0123 activates the check engine light and puts the PCM to failsafe mode, cutting current directed to the throttle actuator, forcing the throttle valve to close to less than 6˚ opening plate. Also, depending on the makes and model of the vehicle, it may show some drivability issues, such as:

  • Hard starting
  • Rough or high idle
  • Surging
  • Engine hesitates on acceleration

Possible Causes

The most common causes of P0123 are faulty throttle position sensor, open, shorted or poor electrical connection at the throttle position sensor. Other possible causes include:

  • TPS not securely mounted
  • Faulty TPS
  • Dirty Throttle Body (or carbon-filled)
  • Chaffed or corroded TPS, connections or wirings

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 on, 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

Error Code P0123 is quite easy to fix, some of the most common solutions are pretty straightforward, These are:

  • Repair any shorts in connections or wirings
  • Repair or replace TPS connector or wirings
  • Replace 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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