I have a 91' loyale with a 93' 1.8 MPFI swap. It failed California smog the other day. It passed at high RPM, but failed at idle. I hadn't noticed it before but it has a slight stumble at idle (vacuum leak?). I sprayed a whole can of carb cleaner trying to find it. Nothing. 

Doing some reading on other possible causes i found that the coolant temperature sensor can cause the engine to run poorly. Anyone have any insight into this issue?

Probably need a new CAT

I don't even know if it has one?

If it does, it's the 25 year old one from the Impreza.

No DEQ (SMOG)  at all here in my part of Oregon, so we never cared about it.

You could try a new set of NGK plugs, and a tank of Premium with some Lucas injector cleaner in.  Then take it for a rippin hot, hard run up some hills right before you go to the test.  Get that CAT hot.

CTS is easy to swap, could be part of it but If it's not throwing a code it's probably fine.  Then again that could be part of the no start that the previous owner thought was an FP issue.

I put new NGK plugs and wires on. I ran sea foam through a tank of gas and some in the oil. I ordered a new coolant sensor. If that doesn't work then it's a new cat.

If the miss doesn’t sort itself out with new plugs and leads I’d then be looking at the coil pack. 

Swap the cat just to be sure. I reckon mine is gone by now, it’s had a few too many knocks when offroad... 

Cheers 

Bennie