I got a second hand Oscar recently.

The guy I bought it from installed a trick switch to make the machine think the water was low and prevent the heating element kicking while pulling a shot. This means that when the the machine is idling the low water light is on and it heats up, but to use the pump you flick the switch, the heating element goes off and you pull your shot.

I couldn't see the point in this, however on the PID Classic thread HB indicated it might be useful:

HB wrote:[A]s we discovered during the research phase of the Buyer's Guide to the Quick Mill Alexia, intentionally disabling the heating element when the pump runs can improve shot-to-shot consistency.

The guy said it would be easy to remove the switch - just disconnect the switch and reconnect the two connectors to the bottom of the water tank. However when I do this the low water light stays on constantly. I've seen this can be caused issues with the seating of the water tank so have tried taking it out/putting it in several times to remove any air bubbles, starting the machine with the pump switch engaged etc but can't get it to function as normal. I connected the two wires directly using a bit of a male connecter but still got the low water light, however using the installed switch I can get the pump working.

I want to try the machine as normal for a period as the way it is presently you can't steam and brew at the same time (runs out of gas pretty quickly). Do you have any suggestions as to why I can't get the machine to recognise the water tank? It doesn't matter which connector goes on which side of the tank, does it? I tried both ways and it didn't seem to make a difference. Also given the pump works using the switch in guessing it's not an issue with the gicar?

if you reinstall the trick switch, will everything
be ok?
if so, go to simonelli website to download the electronic diagram of Oscar to see what you did on it

Yeah when I reconnect the switch, in position a the machine heats up with with low water light on but no pump functionality and in position b the low water light goes off along with the heater but the pump works. Someone on another forum suggested that there looks to be a problem with the low water level sensor.

The guy I got the machine off has suggested following the two wires that go to the tank sensor back to the control box and try reversing them over in case one of them is getting an earth signal when it should be the other. Sound reasonable?

On direct connect Oscars the wires to the reservoir are connected together , and the boiler element will come on during a shot.

when reconnecting the wires did you use female spade connecters ?
on start up , open steam valve , no vacuum release on stock Oscars.
depress brew button , then on button may need a few cycles to lose no water light , and gain boiler element light on , release brew button, when you hear steam like noise close steam valve , wait 1/2 - 3/4 hour

If that works then you might not need a new brain box...

on the brain wires on the three connecter side
white
red
green/yellow

Gluck

the wires have female connections so used a paperclip which didn't work and also tried to use a chopped male connector to join them.

My Oscar has an anti vac fitted.

I tried cycling the machine on and off with the brew button depressed last night but the steam was closed . Would this make a difference?

The pump working with trick switch is confusing me.

Just tried switching the white and green/yellow on the gicar, reconnecting the other end to the bottom of the tank but no luck - low water light stayed on. Engaged the brew switch and powered the machine on and off a few times but no response.