>>>> Update in original post.
what does it do when you try to start it?
The start just cranks it. No sign of anything combusting. When the gas is floored, you can hear a faint putt from the exhaust, but engine starting RPM does not change or fluctuate like a barely-starting engine would.
Sounds like a clogged cat
Interesting idea. Would that really happen so suddenly, though? And shouldn't it still sputter?