Earlier in the year my heated bed failed during every print. The print would get about 75% through and then have a thermal runaway error which stopped the printing.
Looking through the terminal messages on Octoprint it appeared to be an issue with the heated bed losing about 5oC and causing an error. When the issue moved from being sporadic to every time I knew that it was time to have a look!
This is the order that I carried out my diagnostic, which as you can see was probably not the best order!
Step 1: Slice a model with the heated bed not actiavted.
Step 2: Slice a model with a low bed temp.
Step 3: Take off the heated bed, check the thermistor and power connectors. Check resistance of heated bed (1 – 1.2 ohms), check thermistor reading by using a hot air gun at 40oC
All looked good
Step 4: Reassemble printer, level, level again, level a third time, do a test print
Step 5: Check the cables and connector on the printer.
The connector was really hot, one of the pins was discoloured and I burnt my finger.
Step 6: Take apart control box, check every connector. Basically dismantle it. Then try a test print
Step 7: Look again at the connector in a moment of despiration.
I notice that one of the pins is squashed together and covered in soot. I clean it off, clean out the receiving female connector and gently splay out the pins.
Bed heats up, prints, connector doesn’t get hot!
The pin had become squashed at some point, the connection to the power supply was causing some arcing in the very small gap.