When I flush my downstairs toilet, which worked perfectly until a couple days ago, it fills with water to about the level of where the rim starts (no overflow). While doing this there's a very small swirl going on, but no real flush. After the tank empties etc. the water level slowly goes down to where it started pre-flush.

Hi Tanwedar

It sounds like a basic partial clog here.

You need to get a closet auger tool and snake the drain through the toilet. This should solve the issue in most cases... ;)

Mark

I was actually able to solve this without using a snake, yay for free methods. After a few hours of looking around I saw someone suggesting to turn on the tub upstairs and then flush downstairs. For whatever reason this worked and the toilet has been working fine ever since.

If anyone can tell me why this works that'd be great, I'm curious!

Apparently the partial blockage was in the main stack, just beyond the point where the toilet arm intersects. The weight of the water from the tub upstairs and the weight of the water from the toilet flush was enough to clear the blockage. You got lucky, it will never happen again.