I had a similar issue with my first A1 that was still in the return window, so I went that route, so I don’t know what was actually wrong. Note: I am currently working with support on this new issue, but I am not sure if they are on the right track.
So after about 1K hours, I am having a heatbed temperature malfunction again. When it’s heating up, it will get to about 57C or so and then the temp will drop quickly to 0C and of course throw up an error. To me, that sounds like the thermistor in the heatbed is wigging out when it gets hot. But here is what puzzles me. If I manually set the temp to 65C, the display shows both the set temperature and the actual temperature. When the actual temp gets up to about 57C, (just like during a print), it will drop quickly to 0C (takes about 10 seconds). However, the set temperature also changes to 0C, but that happens immediately. I don’t know if that is normal when it is about to go into error or not. Seems like the set temperature should stay the same.
Anyway, I sent support the measurements they wanted and also a video of the above behavior. During measurement testing, everything looked ok. They are saying they want to replace the AC Board, but I am a bit skeptical that’s the problem. Anyone else have the same issue, and if so, what fixed it?
So Bambu finally sent me a new AC board. Of course it did not fix the problem. So I’m back to waiting for them to take the next step. I am 99% sure it’s the actual heatbed (the thermister), because as soon as it starts to heat up past about 55C, it wigs out. One time it went to -22C. If it was the actual heating element, the temp would just slowly drop to ambient temp.
Regarding the set temperature also going to 0C, I think that is normal when it goes into error. What i see happening is the detected temperature rising, and as soon as it starts to drop (wig out), it goes into error. that is when the set temperature resets to 0C.
I’m surprised that nobody else here has had this problem since this is the second unit I have that has. And a Google search shows others having it happen too (mostly on relatively new printers).
In any case, Bambu sent me a new heatbed and it did fix the problem as I expected. My next step was to try and fix the one that failed. I saw a youtube video where someone took the whole A1 printer apart, including tearing down the heatbed. After seeing what the inside looks like, I decided to just cut open the plastic bottom where the sensor is located so I did not have to mess with the bed leveling springs etc… I replaced the thermistor with a 100K ohm NTC 3950 (Amazon 10 for $8), sealed it back up, and now it works fine again too. Guess I have a spare for the next time.