Heat bed remains at room temp until it triggers a heat error

[CombatKarl]

whenever I try to heat the print bed it remains at room temperature when set to heat beyond that. after a few seconds it triggers a heat failure error code. I'm not sure what to do or how to fix this. Any help would be great!

[carefudundae]

It sounds like the bed heater isn't wired in correctly, might have a bad connection, an issue with the control board or firmware, or accessing the wrong thermistor.
I'd suggest checking the following in this order (easy ones first):
1) check to see if the thermistor works and is wired correctly: with the system powered up, but not trying to heat the bed, put your hand on the center of the bed for 30-60s to see if the bed temperature goes up a few degrees... if it does, then you have a good and working thermistor!
2) check the resistance of the bed heater: I can't remember what it is supposed to be, but use a multi meter to check it. The correct value is on this forum somewhere and probably in the manual, too. The best way to do this is disconnect the bed heater wires from the control board and measure resistance between the two. If the resistance approximately matches what it is supposed to be, then you have a properly solder bed heater.
3) if all of that checks out, then make sure you have a 24VDC differential across the bed heater connection on the control board, when the bed heater is supposed to be on.

Start with that and report back. There are a few more things to check, but there is a chance it could be one of those... Best of luck!

[Ron]

Just to add a bit to what @carefudundae said, the firmware in the controller board will register an error if no change in temperature is sensed after some period of time with the heater "on".
This is entirely normal behavior and indicates that your thermal runaway detection is properly working in the firmware, so that is a very good thing.

You did not specify if you have an R1 (12v) or R2 (24v) printer.
If it is an R1 the bed resistance will be approximately 1/2 ohm.
If it is an R2 the bed resistance will be approximately 2 ohms.

The R1 bed heater draws a bit more than 20 amperes when on, which can cause problems on the controller board and its connectors, so look for melted solder or any signs of overheating in those areas.
The R2 bed will draw about half that, but it is still a lot to deal with for the on-board MOSFET, board traces and connectors.

After the problem is resolved you really should consider adding an external MOSFET booster board to handle the bed heater load (even on an R2).
Or change to a mains-supplied (110 or 220 volt as per your local supplies) bed heater with an SSR.
Either way it will relieve the loading on the switching components and connectors on the controller board and will heat faster.

I hope this helps.