its not the pressure the stat uses its the heat as long as the water level is high enough to submurge the themostat then assuming its working it will open etc correctly. If fan was spinning constantly before i'd suggest somthing you've fitted isnt working or you've inadvertantly disconected something when fitting what u fitted. I could be wrong though its only a guess from what you've said obviously!

no there is water in there. maybe there's a slight leak in the system and it cant build enough pressure for the thermostat to work?

I know it prob a bit obvious and you've prob already checked but your coolant isn't low is it? ie not high level enough to affect the thermostat

ttt

fitted a new thermostat when i did the switch.

i've changed the bit that screws into the radiator at the bottom. the other bit was working because it was constantly on before plugging it into the new switch.

could be the thermostat?
Are you sure the fan switch works?

The fan on my car stopped working quite a while ago, so i bypassed the switch and had the fan on constantly with ignition on. Then decided it was about time i got a new fan switch. Fitted it the other day, and the fan doesn't kick in at all. just came home in it and steam was coming off of it because the fan was off.

Does anyone know what could be the problem?

Thanks, Jay