The same happened to me a couple of days ago. I heat gunned a Zephyr mobo, following Wilhelm_I´s tut, put thermal paste and x-clamp replacement. As many other, I couldn´t find the exact set of metal washers, and had to use tiny o-ring´s rubber seals instead oh plastic/fiber washers. Anyway, when I turned it on, it gave me that 0031 code, I figured the heat gunning had trashed, looked for the code meaning and went thorugh some older posts and find out that if you tighten the screws too much or unevenly, the GPU heatsink comes in contact with some of the resistors on top of it, causing that error code.
It could also be a faulty RAM brick, fourtunately it wasn´t, and now my xbox runs once again!!!!!! let´s see how much it lasts.................