Actually, since you have a VS pump, just turn the pump off at the pump control while leaving the box powered. Leave the box switch in the OFF position so that the cell doesn't generate with the pump off. Unscrew the flow switch and manually close it to see if the no flow goes away. You might have to temporarily move the box switch to the auto position, but only do it briefly.
Any hardware store will have a 1/2" PVC pipe thread plug if you want to do it with the pump on.
First, do the test cord. Cut the cord, strip back the wires to expose the copper and twist the copper together to simulate a closed switch.
I will just get the plug as well as I found it.

DURA 1/2 in. Schedule 40 PVC Plug fitting C450-005 - The Home Depot
Use this DURA Schedule PVC Plug in your underground garden irrigation system to function properly. Convenient to maintain.
www.homedepot.com
So if both are successful, then issue is the cell I assume.
Thanks all, I will report back in few hours.