Volvo Repair: Volvo oxygen sensor, intake leaks, smog test

Question
Thanks Roger,
How do they check for an intake manifold leak? And if the manifold is leaking did I need the sensors replaced?
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-----Question-----
My "check engine"light on my Volvo 960 (1997) came on just prior to my smog test. Obviously the car failed. I took it to a mechanic who promptly changed both front and rear o2 sensors ($750.00)as he said a code in his machine indicated faulty senors. While driving back from his shop the light came back on. I brought it back the next day and his diagnostic equipment showed the same oxygen senor problem. Did I just get nailed or is there something else wrong. He showed me the box the sensors came in and they said Volvo parts on the front. Any suggestions?
THe mechanic will have the car again for another day - what should I be prepared for?
HELP - Please

Thanks,
TOM
-----Answer-----
Tom, It may be an intake manifold leak. We have seen this happen where the 02 sensors are trying to compensate too much because of the leak. $750 is a bit steep.  Bosch is who makes those.  Since its in a volvo box they were probably 100 more.  Roger

Answer
Tom, Intake leaks can be checked 2 ways.  With the car running, carb cleaner can be sprayed by each intake hole and see if the engine idle changes.  The 2nd would be a smoke machine.  The sensors probably were bad.  They do go bad.  Its one of those common problems that happens also.  Roger