I own a '99 Volvo s70, with about 70k miles on it, that has recently failed emissions testing with a high co read. The repair shop I take it to wants to replace the oxygen sensors, for a hefty price. Is there anything else that could be causing this? I'd rather not spend over $500 on something that is non-essential.

 

The upstream O2 sensor is a good $150. If you can do car work yourself, if go to one of the parts places you can rent the O2 Sensor removal/installer tool for net $0. The hardest part is usually disconnecting the wiring harness since there is not a lot of room - but I have not done the O2 sensor yet.