After PETG ballooned around my Bambu Lab A1 Mini Combo, I was forced to buy the A1 Series Hotend Heating Assembly. I replaced it into the printer perfectly, but I destroyed the filament sensor. So then I had to buy a spare. It arrived today, I replaced the broken one with the new one, put the printer back together, and it seems to hate me. Every time I turn it on, the nozzle is automatically heated to over 325 degrees Celsius! I tried leaving it on for a minute and it started smoking! All it says is, “The nozzle temperature may be abnormal.” Well duh. It might be an open circuit from the newly installed hotend assembly, but I doubt it. This has gone from “Revolutionary Machine” to “FIre Hazard” in seconds. Can somebody please offer me advice? The universe hates me.
Sounds like you have a short in either your Hot End Heating Assembly or Nozzle Thermistor wiring.
But they are brand new and worked before I inserted the filament sensor. Can you elaborate on Nozzle Thermistor Wiring?
Check to make sure all the plugs are seated correctly.
Specifically #1 in the below picture. This is the connector that has the heater and thermistor (temperature sensor) wiring for the nozzle.
Mine looks the exact same as yours, but here’s what happens after a few seconds: Sorry I can’t put a video down because it starts superheating in seconds.
It went from 84 degrees to 196 in 1 second.
You must have a shortage somewhere. Try unplugging all cables. Then plug the thermistor in again, making sure it’s oriented and seated correctly. Try turning it on; it shouldn’t heat at all.
Now try to add the connections one by one, testing the heating in between and making sure that the cables are oriented and seated correctly as you plug them in. Do not plug more than one in at a time, as this will make it much harder for you to find out where the issue lies.
Good idea! Process of elimination always seems to be the way to go. I just tested and the temperature is still going up with only the thermisistor.
Are you suggesting I pull out the 3 or 4 wire bundles attached to the extruder motherboard?
It’s likely the tool head board has failed. As your description, power is given to the hotend as soon as printer is on.
The only way moving forward is to send support ticket to bambu and get toolhead board replaced.
If for me, I would hook oscilloscope on the gate of the mosfet that sends power to the heater to see what’s going on there.
I like to use mosfets quite a bit and I thought I smelled one burning when the printer tried to destroy itself.
Dang, the same exact thing happened to me on Saturday! Haven’t replaced the hotend assembly yet, since I just got it yesterday. Will make doubly sure not to damage the filament sensor 
Makerworld responded and it turns out they could have shipped me a broken thermistor.
I blame myself for not remembering this: You can tell whether the thermistor is good or bad by measuring the ohms on a multimeter. I do have a multimeter, but it is 30 years old.
I finally got to the bottom of all of this! I was shipped a broken thermistor! After measuring the tabs with a multimeter, it read 0 ohms.
Wow, this is bad luck story. 
Glad to see that you figured it out. I already replaced any part of an A1 extruder in the meanwhile and I can say that having a second printer of same model for cross-testing helps a lot when searching for failure.
Unfortunately, I actually have not completely fixed the printer because of the long ticket wait and it may or may not have damaged some software. The moment it is fixed I will look into getting the multi-plate mod.
It is officially fixed! But the printer seems to like Ludicrous, it just keeps trying to find ways to destroy itself.
From now on, anybody who has a similar problem, please use this topic.
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