Am I being scammed?

I have a 2002 Subaru Outback Sedan H6/3.0 liter engine. My check engine light came on in March. Got a diagnostic and the dagnostic code is: P0031 HO2S Heater Control Circuit Low Bank 1 Sensor 1.

I had the O2 sensor replaced, repaired the wiring to it and added new plugs. Check eng. light came back on. Had Bank 2 O2 sensor replaced because mechanic said that once one O2 sensor is replaced, the other side often goes bad. Check engine light came back on. The same mechanic said that it could be the catalytic converter. He also mentioned it’s a process of elimination to figure out the problem. Since April, 2-3 times I have smelled the rotten egg smell. The smell lasted maybe a few seconds, but it went away. The car was moving when I smelled it.

I just took it to a Subaru dealer to get a free diagnostic. The dealer guy said that there is a restriction in the exhaust and they need to go further (take it apart and charge me) to find out what the problem is.

Can anyone help?

If the potential for a clogged converter is there this can be easily checked with a vacuum gauge.
The problem here is finding a shop or mechanic that uses them and knows how to read them.

Vacuum gauges are cheap, connect in seconds, and can tell a technician many things about what is really going on inside the engine; and the exhaust system.