I have a Taz 6, on my last 30 hour print the item came off looking like the bottom was burned and melted. Tried another print and the bed went up to 149 on the lcd reading. Bed was so hot it bubbled up the pei sheet. Now the temp starts to rise quickly as soon as the unit is powered on and will not stop heating even if a print is started or I use the cooldown command. The temp is set to zero on the lcd, the temp is being read by the unit, as it shows it increasing. I had a second new bed, hooked it up and the same problem continues, so the issue is not in the bed itself. I have reflashed the firmware, reset to factory, all with no results. Hoping for some help here. Thanks
Your F.E.T. for the heat bed has failed/shorted out! You need a New control board. Do not use again until you have replaced it, you can not control it anymore.
Thanks, I kind of figured that a control board was going to be the answer. There goes another 200 bucks. 
Just order the bare board from Ultimachine. You don’t need a kit so, it will only cost $150. Tell them you have a Taz and they will probably throw in the two pairs of headers you will need for the Taz. Or, they might just send you a Lulzbot config board with the headers already installed.