One meaning shows "defective circuit or unplugged/defective fuel injector".

I've had problems with my main relay this summer in the heat... could it be that?

Just not sure what it means by that meaning on that 16 code.

Yes, change the Main Relay, it is the main cause for your code historically, the Main Relay, a two relay assy, is responsible for power to the injectors for half of its job, the other relay sends power to the fuel pump.

. Replace PGM-FI Main Relay.

. Check resistance of each fuel injector, if problem persists.