[westb87]

Originally Posted by Ecky

I believe the sensor is the same on all non-lean burn engines. It's the wide-band for lean burn engines which is really expensive.

My understanding of how it works (and someone correct me if I'm wrong) is that the upstream and downstream O2 sensors just check each other to see that they agree, in a car without lean burn. It's basically either a "yes" or a "no" as to whether or not the air fuel ratio is right, they can't determine what the exact AFR is. If one sensor goes bad or drifts, the two disagree and you get a CEL.

I'm seeing the replacement part as being about $18 on RockAuto, vs ~$130 for a wide band on engines that can do lean burn.

Can confirm. Just bought both O2 sensors for my 04 Civic D17A1. $130 for wideband and $70 for downstream (still haven't replaced the thing!). I bought them from Advanced Auto online and used their 20% off online coupon to order and then picked them up from the store the next day on my way home.

MPG_Guy: I know it doesn't help you AT ALL but those are the miles I'm seeing out of my Civic. I haven't replaced the downstream O2 sensor, but all else in the car emissions wise has been replaced/fixed/cleaned. I'm wondering if your sensors were stuck thinking the exhaust gases were rich, which caused your engine to run lean. Please keep us informed as you have been! I'm in the same boat as you right now, so if you find the solution please share! I'll do the same if I can get mine jumped up. What makes me think my problem could be that downstream O2 is that I'll see random jumps on the SG2 of my cruising MPG @ 50mph around 47mpg for about 1/4 mile or so and then it'll knock back down to my regular ~34mpg while maintaining the exact same load on the engine. I'll keep digging around in the old girl and I'll let you know if I find anything out! Keep on truckin' dude!