1997 Mercury Sable Check Engine Light?

I have a 1997 Mercury Sable with 145k miles on it. The check engine light has been on for years and now that I am driving it regularly I need to get it smogged (which it fails because of light) so I can get it registered.

I took it to a mechanic for a diagnose, and it had 4 codes, all 4 oxygen sensors.

I have had the car for about 75k miles and have never changed the o2 sensors, and do not know when they were changed before that, so I changed them all with Bosch OE sensors, and reset the computer and after about 35 miles the light comes back on.

I took it back to the mechanic, who diagnosed it again, and the same codes came back up. I got some electrical cleaner, sprayed all connections, hasn't helped. I put fuel injector cleaner into gas tank because I was told that could be a problem, hasn't helped.

Anyone have any suggestions for me? Any help is much appreciated, thanks much!