Resetting my check engine light

[joojoo2915]

So I've been trying to get my check engine light to go away so I can sell my car. I have taken it to a mechanic a couple of times and every time I've been told it's the speed sensor. Well the first time I went I was told it was the speedometer head causing the problem, so I bought a speedometer head, replaced the one in my car and no love. Took my car in to another mechanic and I was told the actual speed sensor was the problem so I purchased a speed sensor and just replaced that today. After I replaced the speed sensor I unplugged the battery and put my old speedometer head back in, in hopes that unplugging the battery would reset my computer. Unfortunately, immediately after I replace the speedometer head and have the battery all plugged back in the check engine light comes right back on. It would seem to me that I still have a problem except everytime I have taken my car to the mechanic to have the CEL diagnosed he has reset the codes and it has taken several days to get the light back on. I've also had the light reset itself and it has taken a couple days to come back on.

So, I'm wondering if unplugging the battery may not reset the computer, is that possible? Is there any way to check if I still have a problem without paying another 70 bucks to have the CEL diagnosed again? Any help would be greatly appreciated! Thanks!

Mike