Volvo Repair: Volvo S60 failed emission test, volvo s60, 2002 volvo s60

Question
I have a 2002 S60 (not a turbo) and also have had check engine lights problems. Codes given for emission test failure P0037 and P0128. My mechanic said the rear O2 emission sensor failed and thermostat said engine is running cool. He said neither effect the actual car engine performance. Without paying $550 to get both replaced - is there a way more economical way to cancel check engine light and pass emission test?
The mechanic cleared codes,but the check engine light returned within 15 min. I turned the engine off for about 10 minutes and when I started the engine again the light returned. I had only driven for about 4 miles before I stopped to get coffee. My plan was to drive it on the highway and take it in first thing in the morning but I didn't even get that chance!
I haven't taken it back again to clear codes. I came here to research what other options I might have.

Optional Information:
2002 Volvo S60 B5244S, 2.435 L

Answer
  Amanda, unfortunately, you have no other options. You must complete repairs. I am suprised he said this: "neither effect the actual car engine performance So not true. If he said the car will still run, would be true. Engine must be at the correct operating temperature. This affects emmissions and mileage. Same with the rear oxygen sensor. Even if you took the "bulb" out, the vehicle would still fail emmisssions testing. It will never go into "closed loop" and will not reset the monitors. They check for that at emissions testing.

Mark