2004 Discovery II, 4.6L 62K miles (yes really 62K)
This has been a great truck, no major issues, replaced crank sensor at 53K and normal maintenance.

The bank 2 S1 O2 sensor thew a 0150 code then a number 4 misfire and lean code. Missing, bogging, lurching. I grabbed a new sensor, cleaned up the wiring and connectors, degreased everything and slapped in the new sensor (Bosch). Tadaa, everything worked great, for 3 days. Now it's throwing the 0150 again. On the code scanner (Bosch 1200) I can see all 4 sensors in real time.

At ignition on, all 4 sensors read .455 vdc, Bank 1 S1 & 2 operate normally in response to throttling up and down (inversely proportional) .

On bank 2, S1 starts at .455 then starts slowly descending 440, 430, 400...all the way down to zero over a period of about 3-5 minutes. S2 will start climbing to and level out at .900 and throw the 0150 code again. I've gone through all the air intake cleaned up and tightened. Vacuum system had a few lose connections that I've cleaned up. There's no manifold or exhaust noise, it starts up like new until it starts dropping voltage and I'm stumped. I do notice a little CAT smell (rotten eggs) that wasn't there before, in fact I've never had any CAT odor at all.

The old O2 sensor was shot, and looked like it was in a forest fire hard coated in black soot. Outside of that I haven't been able to find anything on the message boards that presents symptoms in this way. Clues? Suggestions? Wild guesses?

Thanks

 

Every time I have had a sensor that did that, it was a bad sensor. The front sensor goes to zero because it is bad, ECU compensates by enriching the mixture, causing the downstream sensor to go to .900. Swap the sensors left and right upstream and see if it follows the sensors.

 

I'll give that a shot. Hate to think I got a dead sensor out of the box but stranger things have happend.
Thanks, I'll check it and get back to you.

 

Several here have reported bad O2 sensors out of the box, including me. The ones I had that were bad out of the box were all Bosch. What brand are you using? Some have reported better luck with the Walker brand.

 

It's a Bosch, 15175. The ironic thing about O2 sensors are the 61 warnings about careful handling, then you strap it on an exhaust pipe, taking a beating under the engine at 600 degrees. The original was a Bosch too. Fortunately I have a warranty on the bad one and my parts guy said the same thing you did, so he's going to give me 2 new ones to try (one Walker). I'll tear in to it and see which one lives.

 

I got so much practice changing the O2 sensors I swear I could do it blindfolded!

 

they are fairly easy on these vehicles but the driver upstream sensor connection is in an awful spot! I found a matching pigtail that I want to splice in and move the connector (mainly to avoid some leaking oil that finds it's way into the connector) but also to make it easier to swap out but I'm not sure I can get a soldering iron up there to get the splice done well...

 

Once I got the front O2 connectors unclipped from their mounts I left them off so they're now much easier to reach.

There's also a Honda/Acura O2 sensor extension harness available that can be used in a DII. No cutting, soldering and splicing.

Extension Harness 4 Wires for Upstream Downstream O2 Oxygen Sensor Fit Honda Civic Accord CR-V Prelude Odyssey Pilot Acura MDX

 

I learned the hard way to get that offset sensor socket. 8 bucks worth of "save your breaking back" hardware that was well worth it. I'm still waiting on my senors to arrive. but I figured out that my mileage had dripped from 16 mpg to 8. That's a lot of fuel dump for 1 sensor going south.

 

I love my Rovers, but I gotta say, they spent a lot of time figuring out how to make vital components inaccessible. I've started stripping in cable extensions, termination clusters, moved the ignition packs up front. Machined my own mounts for those. 2 of my D2's I pulled the VC's, drilled and tapped 3 more bolts and eliminated the perpetual-oil-puking. They can be a beast to maintain, but I wouldn't drive anything else. (My wife drives an X3 iDrive BMW...when the weather is crap, she comes and gets MY keys) I wish now that I'd photographed and wrote up some of the mods to post here. I'll do that in the future.