Gas furnace safety switches

[Centurian]

I'm trying to get my head around the safety switches on my gas furnace Weathermaker 8000...I understand all the switches (I see 4) are all in series..any one opening will switch off the furnace)

the function of the pressure switch is ..the blower starts up and if it finds a restriction in the flue... pressure will build up in the pressure switch tube and open the switch. is this correct ???
there is one switch I cannot identify and it seems to be in the series safety circuit..I can identify ..flame roll out, high limit and pressure switch ..but what is the last one?? it is situated just as the outgoing flue leaves the furnace thanks in advance

[HayZee518]

pressure switches

usually a pressure switch detects the presence of pressure or the absence of pressure.
in an induced pressure switch there must be a positive flow of air to close.
try jumping our the circuit with a jumper wire from in front of the first switch to the end wire of the last pressure switch or whatever is in line in the series circuit and see how the burner behaves. then its just a case of test and eliminate each individual switch.

[Centurian]

pressure switch

Thanks Hazee..I had it the wrong way round...the pressure switch is normally open and closes on an induced draught caused by the blower motor..showing all is OK