How do I know if my Subaru is in test mode?
The simplest way to tell if the car is in test mode is if the two green test mode connectors are plugged into each other (pictured above). If test mode is active, you can hear the radiator fans cycle, the fuel pump cycle, and there will be a flashing check engine light. Note for JDM ECUs: there will not be a flashing check engine light but the other "symptoms" will happen. Another way to tell if test mode is engaged is if you are getting weird codes like the vehicle speed sensor, starter, and neutral position switch.
Here are codes that will pop up if you check the ECU for codes.
How do I switch in and out of test mode?
This is simple, just plug the two green connectors into each other to put the car into test mode. To take it out of test mode, leave the two green connectors open and unplugged. They will typically be left unplugged as you do not want the car in test mode while driving.
Plugged in = Test Mode
Unplugged = Not Test Mode
Why would I put it into test mode?
The most common reason a Subaru would be put into test mode is to reflash the ECU in a non CANbus car. You can think of the test mode plugs as flashing plugs that get connected when uploading or downloading information to the ECU (usually with a Cobb Accessport). Standalone ECUs like Haltech do not have test mode. For CANbus cars, plugging in the test mode connectors is not needed for reflashing the ECU.
Need to purchase the test mode plugs? Click the picture below.