P0122 Ford 4.6L – Testing a Mustang TPS

Troubleshooting the throttle position sensor in your vehicle manually is not difficult at all, and it’s an important step if you want to resolve the P0122 Ford OBDII trouble code by yourself. The throttle position sensor or TPS is a sensor mounted opposite of your throttle blade, and it informs the Powertrain Control Module (PCM) how much the throttle is being opened. Measuring the angle or degree of the throttle blade opening, and translating that into a voltage signal the PCM can use, the TPS is crucial to your engine operation.

Today I’ll be showing you how to test, service and replace the throttle position sensor in a 1998 Ford Mustang 4.6L GT. To perform these tests on your TPS and find out if your sensor is the problem, or the source of your P0122 Ford code lies elsehwere, you will need a multimeter.



Symptoms of a P0122 Throttle Position Sensor Circuit Low Input


  • Sluggish response from engine
  • Transmission refuses to shift
  • Hesitation or bucking when depressing accelerator pedal
  • Erratic idle or surging idle

The most common of the P0122 check engine code is the MIL lamp or Malfunction Indicator Lamp being lit by the PCM. This is the PCM’s way to request for help or to notify you that there’s something wrong with your Ford.

Where is my TPS located?

The throttle position sensor or TPS is located on the throttle body and it’s mounted to the throttle body shaft. When you press on the accelerator pedal, the cable pulls on the throttle blade, opening it and allowing more air to enter the engine. Follow the sensor by following back the intake tract all the way to the throttle body opening.


When the TPS detects this the PCM can control and adjust the fuel and spark trims to ensure the engine operates at optimum performance. The OBDII P0122 Ford check engine code means that the TPS is sending voltage in ranges that are too low for the PCM to use. You’ve got to take care of this issue if you want your engine to operate and run as it should.

How to Test the Ford TPS for Power


To begin testing your Ford 4.6L TPS, you will need to locate the TPS connector and unplug it. Insert the ignition key and turn the key to the “ON” position. Now using your multimeter, you are going to be checking the brown wire with white stripe or PIN 3 for switched power to the TPS. There should be 5 V of ECU supplied power with the key set to “ON’.



If you have power at this terminal, move onto the ground or the low reference circuit that’s found on PIN 1 or the red wire with white stripe. If your TPS has power and ground, you can move onto the next section where we will measure the TPS signal. If you do not have power and ground here, you’ve got more issues than just the TPS as power and ground are supplied by your PCM. If there’s a problem with the power and ground at the TPS sensor, chances are other sensors are failing or not operating correctly.

How To Test the Ford TPS Signal


The gray wire with white stripe is situated at PIN 2 or the middle pin on your TPS harness. This wire transmits the position of the throttle body blade to the PCM, allowing for the PCM to tune and accurately adjust the fuel and ignition accordingly.


Reconnect the TPS harness and you will be backprobing PIN 2 for the signal from the throttle position sensor. To perform this test, you will need to make sure of a few things first.

  1. Have ignition switch set to the “ON” position
  2. Make sure engine is off
  3. Firmly connect and seat the TPS harness

Backprobe the middle pin while a friend sits inside the vehicle and slowly modulates the accelerator. With the throttle body completely closed, the voltage at this pin should be around 0.8 or close to 1 volt. If you really do have the Ford OBDII check engine code of P0122, this voltage is more than likely lower than 0.8. However if the throttle position closed reading seems correct, we’ll be moving on.

Testing for Wide Open Throttle (WOT)

Have your friend sit in the vehicle and depress the accelerator all the way down to the fully open or WOT position. If you don’t have a friend handy, you can manually open the throttle by depressing the throttle body butterfly with one hand.

Open your 4.6L Ford throttle body to the WOT position or until the bumpstop allows no further movement. Measure the voltage at the TPS signal wire and you should see a voltage between 4.5 – 5.0 volts. As your Ford specific OBDII trouble code is P0122, this reading also should be far lower than what’s expected.

If these readings do not check out, it’s time to replace your throttle position sensor in your 4.6L Ford engine. The part number for this unit can vary because of the number of vehicles this very same sensor is found on. Check below for our listing of part numbers for the throttle position sensor.


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