[Opie]

"U" codes are communication issues, "P" codes are emissions issues:

U0100 - Engine data not received / Lost Communication With ECM/PCM "A" / Missing data for engine
U0164 - Missing data for air conditioner
U0140 - Lost Communication With Body Control Module / Missing data for body integrated unit
U0155 - Lost Communication With Instrument Panel Cluster (IPC) Control Module

P0171 - System too Lean (Bank 1)

For the U codes;
Have you tried to clear all the codes and see which ones return?
Are you sure all the harness connectors are plugged in securly and seated properly?
Were the ECM & Direct injector ECU you replaced new, or used?
If the ECM was brand new, it will need to be set up for the vehicle, available settings are shown in graph below. You will need to specify what market the car is in (Japan,North America,CAN,EU,etc). Once you do that it will ask for other regional settings including A/C type (Auto or Manual), Headlight type (HID or Halogen), Brand (Subaru or Toyota), Transmission type (MT or AT), and more. If these are not set properly for how the car is equipped, you will get odd codes.
To set these up using the Subaru Select Monitor (SSM3):
(a) Connect the SSM3 to the DLC3 and turn the ignition to IG ON.
(b) On the [Main Menu] screen, select [Each System Check].
(c) On the [System Selection Menu] screen, select [Integ. unit mode].
(d) Click the [OK] button after the control name (integrated unit) is displayed.
(e) On the [Integ. unit mode failure diag.] screen, select [Unit customizing].
(f) Use the up and down keys to select each item and apply customization settings.
Pretty sure these setting instructions are similar and can be changed in Techstream as well.

For the P code, check the following components;
• Intake system
• Fuel injector ASSY (port injection)
• Fuel injector ASSY (cylinder injection)
• Intake air flow meter ASSY
• E.F.I. water temperature sensor
• Fuel pressure
• Gas leakage from exhaust system
• Open/short circuits in the air/fuel ratio sensor
• Air/fuel ratio sensor
• PCV valve and hose
• PCV hose connection
• ECM
• Wiring harness or connector