Chosen Solution
Hey, I did an SMC reset but did not resolve the issue. Then I just reseat the cable from Trackpad to Motherboard. Then it works perfectly now. Power on using power button as normal. I found that there is a small dirt particle at the cable connector at Trackpad. Blow that particle off then issue resolved. Poor Apple Technicians. The owner told me that Technicians did try out a new battery, new Megasafe board and concluded that motherboard needs to be replaced. The original owner decided to sell it to me quite cheap.
Sometimes the power button will only work when the MacBook is on or being held down and not pressed, common with minor liquid damage. Check the trackpad board (where the keyboard plugs in) and especially both sides of the trackpad cable (gold contacts). If this is burnt or liquid damage, this is likely the issue. If this looks fine, but a new keyboard, plug it in and test it. If the trackpad cable looks fine, trackpad board is fine and a new keyboard does not resolve the issue, will be related to the logic board so this needs to be diagnosed. My money is on the keyboard :)
have you tried to reset smc/pram?
Was this ever fixed?
