Hi Kavi,
I conducted a few tests that still having the problem. I can only switch on once using D2, after I switched it off, then tried to switch it on again, it refused and the red LED in the GrovePi+ turned on. I recycled the power on the Pi3, the red LED in GrovePi+ turned off, but the relay does not work anymore. Then I connect the relay to D6, I could turn it on and off two times, then the GrovePi+ LED turned on. However, after I recycled the power on the Pi3, the relay could be turned on at least once. I have not turned it off again yet.
It seems that D2, D3 are not working now and I am not sure what will happen if I turned it off now. Please advise. Thanks.
Regards,
jY