Code Killer is a Circuit Board filled with components that defeat Diagnostic Trouble Codes (DTC) - solving up to 9 common DTCs related to Subaru engine swaps. This is optional and these DTC's don't affect performance of your engine, but it is a good way to "clean up" your install and make your Malfunction Indicator Lamp (MIL)/Check Engine Light (CEL) more useful. Your MIL/CEL will only turn on when a critical DTC is triggered. We recommend installing near the Subaru ECU. Code Killer comes with a pinout sheet and you can locate your DTC causing sensor wires at the ECU pinouts page here: Pinouts

For newer Subarus (2002+ Turbo or 2005+ Subaru, Any Model) you can use the Subaru ECU Flash service which will eliminate many more DTC's. See more info here: Subaru ECM Flash

DIY Options:

Malfunction Indicator Lamp: For this code you can just wire a lamp, on the dash or elsewhere. The MIL wire from the ECM provides the ground for the bulb.

Radiator Fan Relays 1, 2 & 3: Wire in some relays, at least for the relay coils, using the ECM fan trigger wire as the ground and ignition power to the other side of the coil. You can also use 470 or 510 ohm resistors, wired one side to ECM and other side to keyed ignition power.

Fuel Tank Solenoids: You'll grab the fuel tank related solenoid wires from the ECM and individually wire them in series through a 470 or 510 ohm resistor. One side to ECM, one side to keyed ignition power.

Fuel Tank Sensors: These ones you'll want to throw on a variable resistor so you can modify their "state". You can use free software like FreeSSM to connect to your ECM and monitor what the sensors are reading (They need to make sense - for example, "Fuel Temperature" shouldn't be reading 800F, make it reasonable ;)

Neutral Position Switch: When using a non-Subaru transmission the best way to simulate Neutral Position is to run the NPS wire from the ECM up to a clutch pedal switch that is either grounded or open circuit (either goes to ground when you depress clutch, or goes to open circuit when depressed). If you don't want to do all that work, you can set up a little transistor (or use the Code Killer) to simulate neutral by taking 12v+ from the brake lamp circuit - this grounds the NPS wire so that it changes state and the ECM doesn't trigger the NPS code.

Rear Oxygen Sensor Codes: Best way to stop these codes from popping is to install a Catalytic Converter and run a rear O2 sensor. If you're not running a cat, then you can space the rear O2 up out of the exhaust bung using spark plug antifoulers to get it out of the exhaust stream. You'll need to experiment and do some more research on the topic as there are somewhat mixed results.