Fan won't go on

[scosgt]

I have a SYM HD200
It is water cooled.
There is a fan in front of the radiator, it is turned on by a thermal sensor that is screwed into the radiator.
So the fan does not come on when the scoot gets hot. It should enable at about 210 degrees F
I pulled the two wires, and jumping them together turns the fan on.
I replaced the sensor with a known good sensor.
No joy, fan does not come on.

So the only thing I can figure out is the fluid in the radiator is not hot enough because:

Air in system (was professionally flushed last month at a dealer)
Thermostat not opening
Water pump not working.

Anything else anyone can think of? I will test the thermo tomorrow by taking off the cap and cranking it up. You can see when the thermo opens.

Any other ideas?