Check engine light is on, but no codes!

I currently have my wr250r up for sale. I took it in to my local dealer (don't get me started) to have it inspected for a potential buyer. The check engine light was on when I brought it in and they charged me $56 to clear the codes (dead battery and tip-over). I got about a block away and the light came back. I went back and spoke with the mechanic and he scrolled through and couldn't find any codes, but he could not get the light to reset. He said the speedo units go bad. The bike is under warranty and they couldn't tell me if Yamaha would cover it or not. Is there something else I could check that would be making the light come on but not throw a code? I don't want to buy a whole new speedo if yamaha won't cover it and it is something simple. Thanks,
Mike

A weak battery will cause the light to come on. Have you checked it?

They checked it at the dealership, but I wouldn't be surprised if they missed something. Wouldn't the battery throw the corresponding code?
thanks! EDIT: battery tested at 12.8v at rest and 14v running. So probably not the battery.

I don't know. I accidentally discharged my battery (while riding) and the light came on. I found the source of the discharge, fixed it, charged the battery and the light went off.