Logitech M705 left mouse button doesn't stay down when pressed

Well the best way to solve this would be to return the device, as it's supposed to have a 3 year warranty.

The microswitch is obviously damaged and all basic solutions would require you to attempt to modify it or its circuit. From your text, I can't see if it shows multiple clicks when the switch itself is held pressed. If it doesn't, then you could try to solve the problem by adding something between the switch itself and the button which would increase the pressure on the switch, but it won't be a long term solution. In situation such as this, the switch is already sparking too much and the contacts are probably already damaged with tendency to get worse so even if this helps, it won't be a long term solution.

If the switch opens even while held pressed, then a little bit of some sort of contact cleaner could be helpful if there's a way to actually insert it inside the switch, but that could be difficult since most of those switches aren't constructed with ability to be opened in mind.

If you happen to have a similar switch, you could try to replace the whole switch, but then you'd probably lose the warranty, in case something similar happens again.

I had the same problem with the left button of my M705 and it turned out that it was indeed caused by a bad contact in one of the microswitches (an Omron D2FC-F-7N), which contained a copper part that was oxidized. I managed to fix this issue by opening the switch and removing the oxide from the contact pad, here's how to do it:

• First, open the mouse. To do this, you need to remove the five screws that hold together the housing: Three of them are located under the black contact pads on which the mouse glides (the pads are just glued so you can remove them by pulling) and two are on the bottom of the battery compartment.

• Open the housing of the defective microswitch by inserting the tip of a sharp knife or screwdriver into the slit between the black, upper part of the housing and the dark grey, lower part, first pushing the upper part slightly to the side and then upwards.

• In the switch there is a small copper plate that connects to a contact pad on the lower part of the switch when the button is pressed down. Remove the plate by gently pulling it first to the side and then up. There is a small bump on the bottom right side of the plate that makes the contact with the lower part of the switch:

  Both the bump on the plate and its counterpart on the switch will probably be covered with a dark, bluish oxide that we want to remove: To do this, use a knife or a rasp to slightly roughen up the surface until it appears shiny and bright again. Then put all parts back into place and close the switch and the mouse.

• Now the switch should again establish a reliable electric contact when pressed down, at least until it re-oxidizes...

Alternatively you could just replace the whole microswitch by a new one, which should cost less than a few dollars but is much less fun of course ;)

Also, before you jump into repairing your mouse check if it's still under warranty and if so consider having it replaced by Logitech, since opening it up will usually void its warranty.

This procedure should also be applicable to other mice that contain the same microswitch, most notably to Logitech G series, Apple's 3G, 4G, 5G series, some Razer mice and Microsoft's Intellimouse Optical.