[Biturbos4] 5 Codes

[Long, Scott G A1C USAF AFMC 88 SFS/S4W]

Recently had timing belt job done, car was running great.  Every couple
weeks it'd throw a CEL for heated 02 circuit failure.  Then it would go
away for a while, and randomly come back.  Lately the light has been on
and the car has been getting horrible gas mileage.

I had 5 codes.  One was random cylinder misfire, one was Lean condition,
and the other 3 were O2 sensor codes.

So I'm thinking the misfires are because the car is running so rich it's
fouling out the plug(s).  The lean code may be because the O2's think
it's lean, but if you look at the soot on my exhaust tips, you'd know
the car is NOT running lean.

I don't know why the O2's can't even last 88k miles, but whatever,
friggin luxury car w/ cheap ass parts.  Have you priced O2's lately.
They aren't cheap.  I am going to do all 4 of them, and change the
plugs.  Might as well.  I hope this is really an O2 problem and not a
problem with the computer or something.

Have any of you used the universal O2 sensors that you splice into the
factory connectors?  They are about half price of the OE style ones.  I
am going to get BOSCH sensors from my local auto parts store.

Are the O2's easy to change on this car if I have an O2 socket?  I want
to do this myself and save on labor.

Scott G. Long