The car is a Mazda Millenia.
Yes, I know I only needed one. The computer only tells you which pair, and then you are supposed to test to figure out which one. The garage was trying to rip me off, it was obvious.
The O2 sensors run between $130 and $180 (memory, too lazy to Google). Dealer charged me about $210 for the part, plug diagnostics to read the computer and then figure out which one. That portion of the work for around $400.
But no, no way am I taking a wrench to my car. I don't know what I don't know, and the cost of the mistake usually far outweighs the occasional rip-off. You have a vast amount of knowledge, and once you get to that level, I think you trivialize it.
When I had my 1977 Impala, I could change my plugs, check for vacuum failure, and open up the butterfly on the carburator on cold start days. I'd do my own oil changes, air filter, etc.
Those days are over.
Last time I did spark plugs was on my Honda Accord. Screwed it up so bad when it got fouled it needed to be drilled out. Never again.