How would this activate a SES light? I do this all the time, is it bad for my 2001 Pathfinder?
I've been trying to troubleshoot my SES codes (PO171-System too lean/bank1, and PO174-System too lean/bank2). Is this connected somehow?

It could trigger a SES light because when the engine is running the tank is supposed to be pressurized, ie, it should have the cap on it. The ECU will see an emissions system failure and could trigger the light. I highly doubt the lean cylinder bank error codes are related.