Can lower back pressure cause a CEL?

Scenario:
It's a '95 to start with (OBD I)
Installed Fister III's 2 weeks ago
Getting intermittent CEL. Doesn't last long, 20 secs sometimes 1 min. Usually at idle or cruising at partial throttle then goes out and my not come back on the rest of the drive.

 

had that with my 95. turned out to be a o2 sensor on the way out. Replaced and no re-occurrence

 

That was my suspicion. That confirms that it's not n "all or nothing" failure.

 

Could be, as well as, might be a sliiiiiiight leak between header and cat.
Before buying an O2, did you unbolt anything from header to cat ?
Just check for leaks.
Another stupid question, did you remove the O2 and mounted back?

The O2 on 993 is heated type.
It takes AIR in from the electric cable connections.
Some guys just unscrew the O2's whilst plug is still hooked on car.
They twist the cables, and the cables will obstruct the air from flowing to inside of O2 sensor after.
just for info...

 

Geolab,

All good points.

Didn't touch headers, just straight forward muffler swap.

I will check connection, but I think it's likely faulty O2