On that boiler the sensors low down are just the old fashioned bulbous type. We've been dealing with thermisters (Negative temperature coefficient resistors) so long I forgot what they are. They aren't vapour expansion because that is used where a sharp response is required, i.e. a liquid when getting hot expands rapidly and manyfold as it vapourises. I think it's just called a thermal expansion system the fluid in the bulb expands and the contact on a stat are made by a bellows of pipe containing the same fluid moving as it expands.
Your boiler doesn't have a low pressure sensor, I don't think so anyway, but the overheat stat would soon let you know the system water had got low.
There are two microswitches, these are made by the passage of water across a diaphragm, which causes suction and movement of a rod that operates the microswitch. One is so the boiler knows when you have run a hot tap, and the other is to make sure the pump works good.
There is another sensor that ensures the fan is working satisfactorily, but on your boiler even if this part is good and the fan is working adequately, the aps might not realise it because a small sense area known as the fan venturi (which has the effect of speeding up a flow of air as it goes through it, creating suction on a side entry tube that triggers the aps) is always geting clogged with this that and the other.
