weather compensation sensor and pump; are they related?

[desktop987456]

Boiler : Viessmann Vitodens 100-w OV heat only boiler.
pump : Wilo variable speed pump;
CH only - no dhw on boiler.
I bought a WC sensor basic (see attached photo) and connected it via provided lead to the boiler on port x21 on board.
The external pump is connected to the boiler on port 20. When WC sensor is connected to port x21 on board, on starting up the boiler, the pump blinks and turns off while the boiler stays in CH mode on the display but it doesn't ignite and stays that way.
If I disconnect the WC sensor the boiler and pump runs fine.
I can't figure out why, I have checked the resistance on sensor and it is within acceptable range ~6 ohms as printed in MI.
Any help to make the sensor work would be appreciated.
Thanks

 

[Johntheo5]

I have one of those pumps and every time they get a run signal, they flash the setpoint pressure for ~ 5 secs and then show the power in watts, W, like yours, all this is telling me is that the pump and presumably the boiler is getting a run signal but only for 5 secs or less.