IAT and PCV Issues with a USDM 2.0 WRX with JDM V7/8/9 WRX and STi Engine Swap

IAT and PCV Issues with a USDM 2.0 WRX with JDM V7/8/9 WRX and STi Engine Swap

Feb 19, '24

In this post, we talk about what happens with the USDM PCV Diagnosis Connector and the JDM STi Manifold IAT sensor when swapping a 2001-2006 JDM WRX or STi in an 2004-2005 USDM WRX. If you need a replacement engine harness for any of these models, we carry it here.

To start we need to define which cars have which parts and what they do. 

USDM - Positive Crankcase Ventilation (PCV) Diagnosis Connector

Starting in 2004, Subaru added a sensor called the PCV Diagnosis Connector. This is a connector attached to a one-way valve attached to the crankcase of your car's engine and removes unwanted gases from the crankcase. The sensor's purpose is to ensure that the valve is attached and the unwanted gases are being removed properly.


JDM - Manifold Intake Air Temperature (IAT) Sensor

JDM STi models have a secondary IAT from the IAT in the MAF (click here for more about the MAF IAT). It's located on the bottom side of the manifold. It's sole purpose is to provide air temperature to the ECU for use with the Auto Intercooler Water Spray. Auto Intercooler Water Spray is a feature found on select JDM models from this era.


NOTE: We do not wire up the Auto Intercooler Water Spray. There should be some information online about this if you want to try it yourself.

What do the PCV and IAT have in common?

The common theme is that they both go to the ECU and get there via pin 13 on the 20 pin main engine connector. This means that depending on the situation there a couple issues that can arise due to small differences between USDM and JDM models regarding these sensors. If you need a replacement connector you can click either of the photos.



USDM Intake Manifold Harness with JDM WRX or STi ECU - Flashing Check Engine Light But No Codes

First make sure you are not in Test Mode. More about Test Mode can be found by clicking here. The 2004 and 2005 USDM WRX have the PCV Diagnosis Connector that goes to pin D2 at the ECU. For JDM models, ECU Pin D2 is used for Read Memory. So, if you use the USDM WRX intake manifold harness and move over the USDM PCV Diagnosis Connector to the new JDM engine while using the JDM ECU, the Read Memory pin is grounded at the ECU which puts the car into a diagnosis mode.

Possible Solutions - JDM WRX ECU w/ USDM Intake Manifold Harness

The simplest solution is likely to just disconnect the PCV connector on the intake manifold harness so the Read Memory mode is not triggered at the ECU. You could also move the pin at the ECU for the PCV Diagnosis to the STi IAT sensor input location.

Possible Solutions - JDM STi ECU w/USDM Intake Manifold Harness

If you are using a JDM STi ECU, you could also just disconnect the PCV connector on the engine. The other option is also to move the pins at the ECU for the PCV to the IAT sensor location and modify the PCV connector. Remember, the only reason that the IAT sensor is there is to provide information for the Auto Intercooler Water Spray system if that were to be installed.


What if I Want to Wire in the JDM IAT Sensor?

If you are using a JDM manifold, you'll just need to move the pin at the ECU.

Pin Switch at the ECU

All you need to do is track down the Blue with Red stripe wire at the ECU on pin D2 and move it to E14. Since the new location is closer to the main harness, you will not have to cut and splice. Please click here for more information on depinning and click here for how to read a Subaru connector's pin order.

If you are using a USDM WRX Intake Manifold Harness - PCV Plug Change on USDM Manifold 

There is no polarity on this connector so if you want to add the IAT sensor to a USDM manifold you can just cut and splice the PCV with the IAT sensor connector. You can purchase that connector on our website by clicking here.

 changes to 

Additional Note About the JDM STi IAT and Speed Density.

Although it is possible to use this input for the IAT to the ECU in a Speed Density or Hybrid setup there are two issues that we note. First is that for the IAT location. It's usually best to put it as close to the throttle body as possible but without being in the manifold for the most accurate calculation.

The Second issue is calibration. We are not aware of any data on the calibration of this sensor so the tuner would have to have this information to be able to scale this properly.