SD2 sudenly stopped heating extruder mid print without warning or kill (Page 1 of 3)

You most likely need to replace the thermistor.
A good way to check this without starting a print and risking a failed job is to heat the extruder and feed 100-200mm in manual command and start gently flexing the wires about. If you get the error while flexing at a certain spot, you found the problem.

thermistor still reads correctly

I had temp screen on, turned on extruder and done the wire moving while watching everything and temp stayed at about 29c

and never could feel any heat on hotend so cant move filament in/out to check it. what volts should I read at wires to hotend?

wen I saw it printing nothing in thin air I looked and extruder temp was about 90c and dropping so I killed job then turnned extruder heat back on to try retracting filament incase jammed but temp just kept falling

sorry my HHHHHHHHHHHHHHHHHHHHHHHHHis sticking

get better service from a $500 TV from China than this $750 pile. dont have $300 for mods now and may as well spend $500 for a China Duel extruder printer with larger build plate it seems since it too can be cobbled on next year. this is why I wouldnt recommend SD in 3dhubs survey

I can guarantee the wife wont let me "blow another dime on this TOY" as she always calls it

Easy way to verify good thermistor or not is to take a hair dryer and blow on the hot end. You should see the temperature go up in repetier-host. If it does, problem is with the heater cartridge. A new heating cartridge (12v) will run you about $8 on amazon, and will include the wiring and everything going all the way back to the boardset. If you really want to make sure the heater cartridge is bad, you can wire it directly to the 12v incoming that goes to your board, then plug it in. The heater will get hot pretty damn fast, so be ready to unplug as soon as you can tell if it's heating up. Polarity doesn't matter.

If the temp doesn't go up, it's a thermistor or thermistor wire. A pack of them is around $10 on ebay, and you can use any wire as long as it can handle the heat. If you want to make sure that it's the thermistor and not the board, you can set your multimeter to resistance and measure across the thermistor. Do the hair dryer trick and you should see the resistance change.

Edit: One common failure point I see for heater cartridges (if they aren't taped properly and secured) is where the wires join the cartridge. There isn't any strain relief there, so if it gets wiggled back and forth too much it will simply snap off. Not much to do at that point except use some high temp solder or replace the cartridge.

thermister reads temp. not zero. and both have power to them but heater dont heat

either heater just went out like the cheapos do or Sheilds filament had thick spot that jammed/clogged and starved extruder over heating it but seems RH limit would have stopped that. its ABS after all not PLA that crystalizes when over heatd which still would only clog not burnout heater. so I say bad heater