91 blazer saftied? help!

i brought my blazer to canada and am trying to get it saftied and e tested. the mechaninc cant get it to pass the etest said someone had removed the o2 sensor so he replaced that and he still cant get it to read the computer right to get it to pass. at 72.00 a hr playing chase games is gettting costly any ideas? thanks in advance for any info on this

Which motor?
Is the Check Engine light on? Any codes?
What is it failing with? High HC's?