The circle button on one of my PS2 controllers was not working, and I wanted to diagnose the reason why. When this has happened to any of my controllers in the past, my first step is always to try to clean it, because sometimes it's just dirt or dust underneath the buttons causing it to not make contact when pressed. In this case, cleaning did not resolve the issue. Typically, my next step would just be to replace the controller membrane, because that is the next most common fix in my experience. However, I wanted to figure out if I could actually determine if that was the problem instead of just assuming that it was. I pulled out my multimeter to test for continuity between the pads. When I started doing this, I wasn't sure what I was doing. Throughout the process I learned that each button pad has 2 separate sides that do not have continuity, and pushing the button down causes those 2 sides to have continuity, and that is how the button presses are registered. I also was able to follow the traces to find that the buttons are connected in a circuit. When testing, I noticed that the right side of the broken button had continuity with the right side of the button above it, but it did not have continuity with the right side of the button below it, even though I believed that it should. When looking under the microscope, I was able to see the issue. At a few different points in the trace between those buttons, I noticed that the traces appeared to have been eaten away. I believe this is the exact cause for why the circle button on my controller did not work.
After identifying the issue, I ordered a replacement membrane online. However, a good rule of thumb I've learned is that nothing is ever as it seems when it comes to fixing things. In this case, something that should have been simple became really complicated, lol. The first replacement I ordered got lost in transit and never arrived. The second one I ordered did arrive, but I quickly noticed something was wrong. The one I ordered had 18 pins, but the one I was replacing had 19 pins. At first I believed I had just ordered the wrong model, but I was unable to find any online that had 19 pins. After doing some more digging, I learned that the 19th pin on my controller does not do anything — I was able to verify this by following the trace and realizing that it went nowhere. So the 18 pin membrane would work, but it was slightly too big. Here's where I messed up. I was cutting around the outside of the trigger traces to make them sit flush, and I was using a really crappy pair of scissors because I was too lazy to get my good ones out of my toolkit. The scissors got stuck, so I squeezed them harder, and then they slipped and I cut through several traces. So once again I was left without a working replacement. The third time I ordered, I ordered 2 in case I had a problem with one of them again. While I was at work a couple weeks later, I got an email that they were delivered. When I got home that evening, they were not sitting on my porch. I was sad and concerned because I thought they had been snagged by a porch pirate and I'd have to buy another one yet again. But luckily, it appeared that the tiny package had just blown away, and I found it in my bushes. After (carefully) cutting it down to size, I was able to put it in my controller, and it was finally fixed!