Sentra XE 2001, 120,000 miles, 1.8 liter, manual xmission.

I can't smog my car here in California. I've had it for 7 years, and this is the first year it was required.

It passes all the smog checks, but I get an "OBD communication failure, " so I can't get the certificate anyway. I need that to continue the registration.

The dealers have tried a "shotgun" appraoch, like "Let's replace this and see it it works." In other words, I can't get a definitive diagnostic from anyone.

It might even be a software problem built right into the car off the assembly line.

I suspect they are hiding something. Is there a way I can see if this problem has been detected by other owners?

A new computer will cost me over $500. Is that same computer used to drive the engine, and regulate the fuel mixture? It seems like is the computer is shot, the car won't even run.