The downpipes on my 3.2 XJ Executive auto were getting very rusty having covered about 108k miles, so this afternoon I fitted new downpipes. I thought it would make sense to replace the two ancient looking O2 sensors at the same time, using good quality NTK items.

Got it all back together and was feeling pleased with my self.... Headed off for a test drive and was disappointed to find that the car was now idling slow and stalled a number of times in traffic....

It's a UK spec car, so it just has a pair of O2 sensors, both downstream of the pre-cats in the downpipes.

Is this something that I need to wait for the car's ECU to adapt too?

Thanks in advance for any input.

 

Is there a chance you may have reversed the connections for the two sensors?

There are threads about O2 Sensor "orientation" that needs to be done with Jag Diagnostics (don't know if that would cause your symptoms) but there are also examples of people who just change the part with no issues.

 

Thanks for the reply.

I may well have reversed the connectors, I did not record which was which before disconnecting them when I started the job.... Stupidly I didn't think it mattered, because they were both downstream of the precats. But thinking about it now, I realise that each O2 sensor is related to 3 specific cylinders and if the connections are reversed they will feedback the wrong information to the ECU.

I will try swapping the connectors over this morning and see what happens.

Will report back later....

 

In addition, there is a fuse for O2 sensor heater.
If it blows, it takes time for the sensors to warm up, thus the voltage reading will be out of range until they warm up completely.
You may want to check that too.

 

Well, I swapped the O2 sensor connections over but the motor still seems the same, running a bit rough at low speeds, idling slowly and tending to stall.... I need to investigate further, it's very frustrating because the car was a sweet and reliable runner before yesterday's work.

The only other thing I did to it was to change the auto box fluid while the car was up on ramps. Selection of drive/reverse seems a bit sluggish now too, presumably due to the slow idle I am guessing....

 

I hooked up my AE ODB device today to see if it would shed some light on the rough idle / stalling situation (Screenshots attached)

No codes thrown up, but it did show something going on with the O2 sensor on bank 2....

High voltage reading and very negative STFT reading.... considering the car was running great before fitting the new O2 sensors and downpipes, does it sound like one of the new sensors is duff?

 

The readings would suggest to me that the O2 sensors are reversed and therefore the voltages are pinned high and low because of rich and lean mixtures in the 2 banks as the ECU is trying to compensate for a condition that is actually in the other bank. The STFT is showing that also, pinned at +24 and -24 on the two banks.

 

Thanks Brendan, I did swap them over already after your suggestion a few days back but I stupidly did not take any OBD readings first.

I will try swapping them over again and see what effect that has on the readings.

 

A few days back I swopped the two O2 sensor connections over and the car started and ran smoothly. Seems like I must have mixed them up before....

Then I took it out for a 30 minute test drive with varying speeds and throttle loads.... It idled smoothly and ran like a dream again. An added bonus has been that the average fuel consumption has improved by about 3 mpg since I fitted new downpipes and O2 sensors

The transmission needed a top up too, I have set the fluid level with the engine/autobox hot, all seems well with that too. The shifts are noticeably smoother after the fluid change.

 

I also rechecked it with my Autoenginuity OBD device by the way, fuel trim etc is now showing as being evenly balanced

 

Good news rowland and thanks for posting the conclusion. It's surprising how many people don't!

 

I know, it's frustrating how many threads just suddenly end!