Spray your wires down with "pest away" or other such deterant you can buy at a pet store...often the scent of mongoose urine that small furry chewing beasts intuitively know and stay away from. No sense in wasting $$ from rodents and what-not's chewing up your electricals. Other option is to wire in an ultrasonic "ear buster" close to the source of concern and have it running off of your battery...shouldn't pull enough amps to drain it, I wouldn't think. A switchable unit that you can plug up to 110AC would also help when you're away for a long time. This last choice could be problematic if you have dogs travel with you in your Volvo. They would probably drive you crazy from their painful whining. But it's a trade-off...put up with your dogs whining or have to pay $$ for 02 sensors several times a year.

On a more serious note, I think you might consider finding another mechanic. In my opinion these items don't fail so often and I'm thinking you're being taken for a a very expensive and frustrating ride...sort of like I did in my first paragraph above.
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1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.