Volvo Repair: emissions, oxygen sensor, turbo wagon

Question
1987 turbo wagon, 46578 original miles, I failed to co (during drive) on the emissions for Mass state inspection. maximum allowed 30 my car 85.00  I have just spent 250.00 replacing pressure regulator (they said that was the problem), then full tune up (that was the problem).  I just had it retested and now testing at 108.48.  I don't want to keep replacing things mainly money reason I thought it was the oxygen sensor but when I suggested that they said there was no problem with that part.  Can you think of any ideas so when I bring it back I won't be replacing things like above that aren't going to help the problem.  I just bought this car and really like and would hate to have to get rid of it because it doesn't pass emmissions.  Any ideas would be greatly appreciated.

Answer
there are too many things to just guess.each sensor should be checked to determine the problem.sorry I have no easy answer.I would try the dealer if you have not been there.if it is the dealer then keep on them to fix it and that you will not pay for any repairs that do not fix the problem.