[Update] Thanks to reader Andrew Mikolajczyk who posted another code that is available. See it after the break along with the rest of the codes.
Like any mobile provider, Project Fi has a list of Dialer Codes that force your Nexus device to interact with the service in a certain way. This could be, for example, to force your phone to only use T-Mobile, or Sprint or even repair a bad activation. Generally speaking these codes should only be used in specific circumstances with the guidance of the Project Fi support team. However, if you want to try them out, you can do so and you will find all of them after the break.
A word of warning before you jump ahead. While I am publishing these codes, Google can change them and I make no guarantee they will work for you. Further, you use these codes at your own risk. Dialer Codes will force a behavior and to “unforce” that behavior, you may (likely) have to reboot your device. Note also that these codes are aimed for Nexus devices. Given that Project Fi is not officially supported on any other devices other than Nexus devices (6, 5X and 6P), your mileage may vary on other devices you have connected to Fi. It should work but again, no promises, guarantee or warranty implied.