Bathroom stench after flush and chemicals

Here's another path to go down, because it happened to me. It wasn't the toilet at all, but the stink was coming from the bathroom sink. Underneath our bathroom sink, there is not a vent that goes directly to the roof. But there is an "air admittance valve" which is a perfectly fine way to vent the plumbing. But our air admittance valve failed, and it was letting tank gasses come into the bathroom at all times. I could have sworn that smell was coming from the toilet, and accused my son of covertly eating pickled eggs and cabbage.

If it is the air admittance valve on your sink, it's very easy to fix. Just unscrew it, and buy another one from Lowes or Home Depot for about $20 and replace it.