Here's some information about it.
Your 1.8 liter engine is probably the same as our 2.0 liter engine.
It's not good news.
https://mazdas247.com/forum/t/what-have-you-done-to-your-p5-today.123616046/post-6733787...
I read that a guy replaced his seals for good measure and they started leaking, then he ended up having to replace his seals almost yearly from then on, so I didn't replace my seals when I did my timing belt.
My seals weren't leaking, so I left well enough alone.
Apparently there are better...
You might get lucky and clear that code with a new upstream O2 sensor, but that O2 sensor is there to monitor the condition of your pre-cat, and p0421 is supposed to mean that your pre-cat needs replacing.
There is a non-fouler hack to get rid of that code without replacing your pre-cat...
That switch has nothing to do with the air bags or seat belts. It simply turns on the idiot light to remind you to put your seat belt on.
It also has a timer circuit to ring a beeper for a few seconds.
This is what it looks like.
If that light comes on and won't shut off, you can turn it off...