So to start with, you mentioned the underextrusion on a previous print. Seems like a good starting point, when was the last time you checked your E-steps? Basically you want to disconnect the bowden tube from the hotend, extrude out a short amount of filament and mark its position, then extrude 100mm of filament and measure how much actually came out. From there, there is a formula to adjust the E-steps on the printer. Ideally you should have exactly 100mm come out but there’s a good chance you’re going to have less than this. You can also make some adjustment to this from your slicer (in the material flow section) but that can cause various other problems, so ultimately you’ll want to get this value corrected in the printer itself.

While the bowden tube is disconnected, this is a good time to try doing a cold-pull. Heat the hotend up to around 200C again, stick some scrap filament into it so it just starts to push filament out the bottom, then let the hotend cool back down to near 160C (or maybe even cooler, but this is a good start). Pull the blob of filament out of the hotend, and you should have a bullet-shaped plug on the end of it. Look this over to see if there is any burnt filament, contaminants, or anything else that looks weird. If you see obvious contaminants then this is likely causing your underextrusion. After doing this, you should also check the nozzle itself, sometimes as they wear out a bit of the brass gets pushed over and blocks the flow. Always keep spare nozzles on hand, they wear out faster than anything else.

And one more thing before reassembling… Check the extruder itself. After some time it is common for the brass gear to get clogged up with filament or simply have the teeth wear down, especially from some of the fancy filaments like wood, glow-in-the-dark, or even the metallics. However the results of these problems should be fairly obvious from a clicking in the extruder while printing. Clean out any obvious filament remains, or you can get a pack of replacement gears pretty cheap.

When you are ready to reassemble the bowden tubing, check the fittings at both ends. These wear out easily, so you may see signs that the tubing have been shifting back and forth. These really need to prevent any movement in the bowden tubing, so if you’re going to order parts anyway, get a pack of these to hand on hand. Bad fittings can cause serious underextrusion any place the extruder reverses directions like at the end of a wall. but the wall itself should lay down fairly cleanly.

Hope that gives you some ideas to run with. Some of this will depend on the specific model of Ender you have, but if it was working fine and just suddenly started having problems then something blocking the filament flow is at the top of the list of possibilities.