Hi all I had a major block in my nozzle and the filament broke off around 4 mm into the top of the heatbrake. I heated the nozzle to 250 and tried to push the filament out and it would not budge. I then set the hotend to 180 deg C and heated up a small allen key and stuck the allen key down the heat break throat firmly into the filament. Once the hotend settled at 180 deg C I pulled the entire section of filament out in one piece. I now heated the hotend to 250 deg C and pushed PLA filament through the hotend manually unit all the old white filament that was stuck stopped showing. I then switched the machine off and proceeded to refit the hotend and all of the trimmings. When I switched the machine on again I got an error showing the hotend was faulty and the Lidar would not work. I checked all of the connections and all appeared to be fine. Ran a self-test and nothing happened. Fitted a spare hotend assemble with the same result. The temperature shows 18 deg C on the hotend when the machine has booted up and then gradually drops down to 0 deg C. Any ideas what I can do to sort this out? Please assist if anyone has resolved a similar problem.
#1 if you have the printer on when you unplug the hotend wires you will at the very least get a thermistor error which is an easy fix by turning off the printer for for a minute then on again
But reading your query it sounds like you did all of that
Soooooo, now we need to make sure the JST plugs are not plugged in backwards (it can happen easily)
If that all checks out make sure all the pins on the board for the hotend are all good
If all of that checks out we’ll think of something else, but first things first we will keep it simple
If you need to go to the next step
On most electrical circuits with a NTC sensor when the circuit goes’ open they will go to a low value.
If you have a multi meter you can check the interface board ntc pins if the solder joints have cracked this is a photo showing V8 board they also have a V9 board but same way to test it.
Also some people have had the tool head board just fail for no reason.
The nozzle temperature is abnormal
The micro lidar try cleaning the lens Micro Lidar Clean
Keep us updated 
Thanks for the feed back I am going to check if any of your possible faulty areas to check if I missed something. Thank you Jrock and 3DTech