Creality K1C after tightening the Heat bed screws which was 1/2 turn loose .
Got parts to replace the Leveling Sensors, About to replace it only to find the screws were loose so I tighten them tried to print new Filament ,Tweaking the setting because of sticking issues. finally got it set and this happens…
You may find the answer you need here
I found the 2121 error code almost at the bottom of the page. It looks like error 2121 is related to Gcode.
Hope this helps 
The bed screws top side needs to be snug, not tight, to allow the bed to flex/move a little during heat-up. If not, you may end up what Voron builders call “Tacoed”; rigid mounting can result in this happening to a 6mm thick aluminum bed as well as a 2mm thick Creality stock bed.
On the bottom side: The spacers to the screws through the load sensors should be tight but not over-tightened. This might damage the load sensor. That error might be that result.
i think it may have to be tight I had the K1C and the mount is very different to the K1 MAX which that part sent may not work. part numbers don’t match…
does not use a 11mm nut at the bottom . it’s a hex screw to the spacer
Follow up .
Installed the K1 MAX Sensors and board(same part Creality Ender-3 V3 (Regular) so far everything is working Great …
