The OBDII DTC trouble code of P0121 is a common problem that relates to your Subaru throttle position sensor. When you depress your accelerator pedal, the attached cable opens your throttle body. The throttle position sensor is just a big potentiometer that reads how far or how little your throttle is open.
The throttle position sensor in your EJ20 engine informs your Subaru ECU of the exact positioning and degree your throttle blade is open. The part number for this throttle position sensor (TPS) is 22633-AA140 as well as 22633-AA210.
Today I’ll be showing you how to change a WRX throttle body to correct the OBDII check engine code P0121. This code can also come up with other related OBDII DTC trouble codes, like P0122 or P0123. The exact language of the P0121 OBDII trouble code is Throttle position sensor circuit range/performance problem (high input).
This means that your Subaru ECU has received a value that’s outside of the normal 0-5 volt operating range. This can mean that your TPS has slipped or become damaged, or your wiring to your sensor is bad. Today I’ll be showing you how to change a WRX throttle body in a 2002 Subaru WRX with a EJ20 engine in it.
Before you begin our how to change a WRX throttle body DIY article, disconnect the negative terminal to your battery.
As always, when working on a My Pro Street DIY article always work with safety in mind first. If you need a few pointers on how to work on your car safely, check our Safety Guidelines here.
You should work on your Subaru WRX when the engine is not warm, as you will need to drain coolant. When you are ready, locate your throttle body and begin disconnecting all the electrical connectors that lead to it, including your Subaru TPS.
Now that you’ve got your Subaru Throttle body unplugged from your WRX engine harness, you can slip off the coolant ports. These ports help the throttle body during extreme weather and prevent the chance of ice build up or a frozen throttle body. These coolant lines can be warm or hot so be sure to cautiously disconnect if these hoses are warm.
Once these coolant lines have been removed, you’ve got to undo the worm clamps that connect the throttle body inlet to your top mount intercooler outlet. Disconnect your MAP sensor, throttle position sensor and Idle Air Controller, that’s on the right hand side of the Subaru throttle body.
Carefully undo the worm clamps to the throttle body coupler, and remove them. You can take one of two options here at this stage, you can opt to slide off the coupler by pinching it closed to fit through the gap between the intercooler and throttle body. Or you can simply disconnect and loosen the 10mm bolts that hold your intercooler in place, and push it up gently.
If you’re going to be moving your Subaru EJ20 intercooler, make sure to disconnect the blow off valve recirculation outlet, shown here. Compress this clip and slide this BOV discharge hose off the Blow off valve.
There’s four 12mm bolts that secure the EJ20 throttle body to your Subaru WRX intake plenum. Once loosened, carefully open your throttle body and slide out your throttle cable end, this will release the throttle body. Here is where you can complete our How To Change a WRX Throttle Body DIY article, by simply replacing the throttle body now and clearing your code using a scan tool.
If you wish to test your Subaru WRX throttle position sensor connector, you’ll be testing the wiring harness side of the TPS.
With the EJ20 TPS harness facing towards you, you will be testing the left most pin or PIN 1 for switched power with the key in the ignition and turned to the ON position. The low reference wire is the third pin from the left, or PIN 3 on the three pin Subaru TPS connector. The middle pin is your signal wire, which should present a value between 0 – 5 volts from closed to wide open throttle respectively.
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