Cherokee Sport 2000. Couple months ago had an O2 sensor error for Bank 1 Sensor 2. Engine running OK. Cleared the code and was fine for awhile then it reappeared. Cleared it again, but decided to take a look see at the sensors. Found:



and it seems to run fine! Apparently even though it sat at 0.4 volts, since that's within normal operating range the ECU don't care!

Weather is better up here now so changed it out:



cause I figured if it quit altogether I might end up in a bad spot.

The new sensor (NDK 23110) signal looks a little calmer than the other sensors. Wondering about that. I would've thought it would be the other way around, new sensors would be more sensitive.

One also wonders if waiting for the CEL to go on is necessarily the best way to check sensor integrity. Then again, perhaps a flatline sensor output really doesn't mean anything.

 

"Seems to run fine" I think is the key statement. You might not notice if your sensor is bad, it just may screw up how the computer handles the engine and incorrectly determine functions leading to less than efficient operation. Or being that it's CA emissions, maybe the additional sensors are just to make the hippies feel good...

 

Its because it is a downstream sensor that is used for affirming that the emissions system is working correctly, but it is not used by the computer for making air/fuel adjustments. The upstream (bank 1 sensor 1 and bank 2 sensor 1) O2 sensors are used for the air/fuel adjustments. So when those go bad, they typically express themselves with poor running engines (lean, rich, etc).

 

Thanks for your input, Jordan. Continuing to research the role of the post-cat sensors, it seems variation in voltage suggests a failing cat. IOWs my initial flat line would appear to be good and I should have left it alone! The old B1S2 sensor did not look bad on gross examination. I am stumped as to why it's output was stuck on 0.4 volts. It would seem to me that a failure would be 0.0 volts and that was that.

I also wonder if post-cat sensors DO contribute to AFR as Bank 1 now appears to be running decidedly richer. Mayhaps I shall drive around a bit and do some live monitoring. The engine was not really warmed up when these measurements were taken so I'll give ECU some time to think about it.

Also, recently was getting REALLY bad MPG, pulled and cleaned the injectors. Seem to be getting good spray now and MPG is back to normal.

 

Thanks for your response, Jordan.

However, in the above graphs B1S1 sensor says that the AFR has become richer with the new B1S2 sensor. Does that not suggest that B1S2 has at least some contribution to AFR?

 

Those graphs are a relatively short snapshot in time. Things might have very well been running slightly richer when the second graph was grabbed. Its not a lot richer, I see quite a bit of scatter on both.

But as to the question you asked, I am partly basing what I said on my own observations, and partly on what I have seen other knowledgeable forum members say before. Also I have a 96 with 2 sensors only (one upstream, one downstream). So it is possible that I am wrong (either entirely, or just for the 4 sensor arrangement). If I can confirm what I'm saying is right/wrong via the FSM (or some other gppd source) I'll come back and mention it.

 

I will say that my upstream sensor was bad when i got this XJ, and it let me know it every time i ran the thing once it warmed up.