I've been getting a 4-3-5 code for a while, been resetting the codes but they keep coming back on after a few weeks. I've been told the front o2 sensor is at fault, but read that I should replace both regardless. Any 850 owners with opinions on this?

Warmest,

Ron

Mine were old and was giving the car a complete going through. Did both with genuine good stuff. If and when you do replace, I would do both. Not a bad job except on the pocket. Make sure you cat is good too.

dick

I wouldn't replace either one, and no, they do not come in pairs nor do they fail in pairs. As expensive and troublesome to replace, I would look for other potential problems first.
Check for vaccum line leaks or blockages.
If you have been using El Cheapo gasoline, get a couple of tanks of good gas.
A good tune up is always worth the expense.

Klaus
--
1975 164 w/174,800mi (Sold) 1995 850T w/91,000mi, 1998 V70R w/129,000mi

I second this - also a small exhaust leak (loose clamp,etc) can trigger O2 sensor codes.

Thanks for the replies. I had the car in the shop a month ago for a fuel pump replacement and asked them to check for vacuum leaks on the side, but they didn't report finding any. I know this is a newbie question, but is this check something I can easily do on my own, without any tools or putting the car on ramps?