Like many others I got this error after several months of use of my SOVOL SV06, indicating a faulty thermistor on the heat bed. (This issue can also happen if you try to start a print with the USB connected to a computer but forget to start the power supply…this is not that problem)
HOWEVER, after disassembling the heat bed and seeing that the thermistor measured fine at 115k ohms with a multimeter, I suspected the problem was with the wires themselves. The problem was in both yellow leads to the thermistor as they had no continuity on the multimeter…and after dissecting them very carefuly, I discovered that both copper wires leading to the thermistor had broken in approximately the same spot, about 4" from where they were soldered onto the bed. This is exactly the location that the cable that leads from the hotbed to the control board kinks when the bed is moved to the back of the printable area. It appears these copper cables have failed due to metal fatigue of being flexed at this single point over and over again.
My solution was to replace the copper wires to the thermistor with a wire from an old USB cord, which is made of copper coated steel, and designed to bend and flex throughout its lifetime and not fail mechanically like these copper wires did. I don’t expect I will have to replace this cable, so I’m posting here so that others might also benefit from this enhancement, if you aren’t afraid of a little solder / splicing work. A new heat bed is only $30 as of this writing, but most people have spare USB cords of all kinds laying about, so this might save people a few bucks.
SOVOL: swap out your copper wire leads to your thermistor with USB wires in your manufacturing process and you will cure many issues with unresponsive heat bed thermistors from customers!