Civic loses power when 2 cold or rains and failed emissions

Hello :) My boyfriend has a 1996 Honda Civic CX (236,000 kms) 5spd, 1.6L, 4cyl and when it's very cold or after it rains it sometimes hiccups and loses power, especially on the highway. Aside from that, the engine light is on and its failed the emissions test with a too high concentration of co2. We have been told it may be the oxygen sensor, but it was changed last year and it was very expensive. How long are they supposed to last? Hope you can help. Fingers crossed that the solution is inexpensive because we don't have a lot of money right now (he's in school). Also, why do part stores and Honda repair shops charge $260 for an oxygen sensor and I was able to find one on innerauto. Com for around $50? And Partsource sells them for $116-$225, what's the difference? Thank you SOOO much for taking the time.
Oh, the car has had a tune up last year, timing belt changed the year before. Thanks!