Hello all, have a X1 Carbon. I have never had any issues printing over the past 2 years. Have done all the maintenance and updates, then all of a sudden the prints would start to fail after the reach a certain height. Using bambu labs PLA for my prints and a two AMS’s.
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First Pic is the lower side, Second is the same side only further up, third is the top
Heat the nozzle to temp and with the teflon tube remove check if you can hold the filament while extruding.
You clearly loose the extrusion more and more and I suspect a heat issue.
If the extruder motor isn’t struggling it might be worth to check for build up on the transport wheels and whether or not some teflon tubing is worn out.
Textbook underextrusion, because of the sensors on the x1c (should rule out extruder issues), I would start with the nozzle - Have you tried swapping the nozzle, if it immediately goes away…
My guess is something foreign is in the nozzle, all it takes is a fleck of something in one of those spools to block it just a little bit for this to happen
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Thanks for the help, just replace the hotend and now the test cube is perfect. Ordered a another spare.
Thanks again
When you replaced the hot end, was the malfunctioning nozzle bent?
You seem to have some elephant foot and warping initially while later you have clog like underextrusion. That can indicate a curling problem which may reoccur if you keep your settings. To reduce curling, print with a lower height and default nozzle temps.
I know it is a bit off topic but since the hotend was replaced…
You can remove the sensor and heater to be left with just a metal piece - one you declared ready for the bin.
It is a good idea to try the hotend one last time in order to pull out the filament properly.
After that take it all off and toss it into some good solvent for a while, like a few days and prefer to use a sealed jar or such…
Acetone is always a good start but even plain Ethanol wrecks most filaments after a few days and turns them into goo.
MEK as used for priming PVC pipes before gluing them is a great option as well.
After that you can always try nasty stuff like Methylene Chloride or just check the WIKI for the material in question to find suitable solvents.
Do not rush things and try to also have some small jars or such for filament samples with the solvent in question - you can toss short pieces of all your materials in the same jar…
When the samples turned into slimy goo chances are the inside of your hotend looks the same now.
What to expect to find ?
With the nozzle off you probably find sticky goo but sometimes also flaky stuff.
These flakes are over baked filament that basically turned into charred remains - they love to come loose eventually during long prints to block the nozzle more less.
I prefer to use some string made out of cotton balls to clean the first good out then I use a pipe cleaner.
Not, not drain cleaner, these thing thingies for old school pipes your grandfather might have still smoked - these days used for all sorts of things and available in hobby and $2 stores.
Be aware that those are not solvent resistant so you need to make sure the inside at least got a quick IPA or Ethanol rinse as otherwise you are just left with a wire.
With those measures most hotends are good to go again - most…
In some cases you will signs of wear and tear - if you see crack lines inside the melting chamber or elsewhere inside the filament pathway toss it out…
If it no longer looks shiny or has some minor scratch marks it means there will be a lot of resistance for the filament and a lot of areas where it can bake on.
Use some metal polish paste/creme and a cotton string or strips and if you have a friend with a lathe ask for help 
A lathe makes it very easy to hold the hotend while it rotates so you can use a toothpick with cotton around it such.
The other way around with the toothpick in a slow running drill works as well but try to put the drill in a vice or clamp it to the table.
I know what some will say now - WHY BOTHER ???
Well, no everyone has the luxury of full Bambu support and availability…
In some countries you pay about as much import tax as some parts cost - if you can even order them.
If then you can revive a hotend it means you have a working spare or that you can keep printing.
Having some spare nozzles at hand is always good, same for some dental wire of the right diameter to check how worn out a nozzle is - once a 0.4mm wire just wobbles loosely in the hole it’s time to take a new nozzle…
Just saying…