I have been working on a check engine light indication for the past several weeks. It started with a P1646. On my 1999 XK8 this is the Bank 1 upstream oxygen sensor. After some testing with my trusted technician (I was in the garage for some other items) the heating element tested bad and he replaced the sensor. (I let him do the replacement, after all he did the testing.) Repair work is done.

I am still getting code P1646 so I did some further testing on my own. I first tested connector position 1 and 2 on terminal EM021 which is the terminal connecting the sensor to the ECM. The resistance on this terminal for both Bank 1 and 2 are exactly the same.

I connected my diagnostic terminal and got the following data:
Bank 1 Sensor 1 Equivalence Ratio 0.999424
Bank 2 Sensor 1 Equivalence Ratio 0.999424
I was surprised to see them exactly the same, but

Bank 1 Sensor Oxygen Sensor Current 0 ma
Bank 2 Sensor Oxygen Sensor Current -0.1484375 ma

I am worried that Bank 1 sensor current is 0 ma. I thought it should at least be something! But, not sure.

Any ideas from anyone would be helpful as this is getting frustrating.

 

I think it's possible you may have gotten the wrong upstream O2 sensor. Your '99 should have a wideband sensor, and if you got a standard zirconium sensor you could be in this predicament. Beginning in '99 the NA XK8 has the AJ27 V8, and uses a different sensor than the earlier cars. I've found lots of issues in the aftermarket regarding these sensors. Have the supplier check whether what they gave you is not also specified for a 1998 car in their books. If it is, it's wrong.

Good luck!

 

I did some quick checking on the web for the sensor. My R.O. has the part as a 854559A Bosch Bank 1 Upstream Oxygen sensor. When I search for the sensor generally on the web it references Bosch, but part number 15048. Using the Bosch part look up capability it uses the 15048 for both 1998 and 1999. I spoke with the tech this afternoon and he understood the difference between years that I mentioned. He said he was assured by the parts store the sensor was 1999 only. I think they were not exactly truthful with him. I am headed into his shop tomorrow am to follow-up and he might have the same conclusion that I reached as a result of Indy Steve's comments and my research on the Bosch site. More tomorrow....

 

I just realized I had not closed out this problem with a proper reply. Several months ago I suggested, after Steve's email, that the O2 sensor had to match the year for my '99 and not be a multi-year. The garage said the parts store had indicated the sensor would fit multiple years and insisted the part would fit the 1999. I offered to buy the sensor from the dealer and if it did not resolve the problem, I would buy both sensors, no questions asked. Guess what? The dealer sensor fixed the problem and it has operated as designed ever since. No error codes. It worked out in the end. The garage insisted the parts house take the multi-year back because it did not work as described. Everyone ended up happy. It just took a long time.

Thanks Steve for the idea.