Hi there,
I've just replaced my 2 upfront O2 sensors because of engine hesitations under load at low RPM, but after replacement it got much worse (many random misfirings and a loss of power even at high RPM).
I cannot see the O2 sensors output with my diag tool, but I can see the STFT. With one of the new sensors, the bank with the new sensor has a very ample STFT control and gives engine unstability. The picture shows bank 1 with an old O2 sensor, and bank 2 with a new one (aftermarket).
Anyone knows what it could be?
Now I've put back my old O2 sensors and it's back to the presvious situation, which still allows me to drive.
Cheers
Francois

 

What brand of sensors did you buy? Do you have trouble codes to go along with those misfires? If yes, look them up @ https://www.obd-codes.com/trouble_codes/

 

Don't look them up there. Some are wrong and the others fail to list all the causes for the specific car.

Instead, use the jag info that's a free download!

It is both correct and lists the causes for each code.

 

Hello,


the original ones are DENSO 2R83_9F472AC (made for Jaguar C2C29250), but I've bought 2 aftermarket cheap and no branded claimed to be at the spec for C2C29250.
They claim it compatible with all these models:
- XJ6 XJ8 XJR 3.0 3.2 3.5 4.0 4.2 (1999-2008)
- X-TYPE 2.0 2.5 3.0 V6 (2001-2009)
- S-TYPE V6 V8 2.5 3.0 4.0 4.2 (1999-2007)


I'm returning these right now.


Also, another tip, it is that when I changed in the same time my old spark plugs (were Iridium) with spec ones I got many more misfirings, so I put back the Iridium ones.


I've no fault codes, I think the misfiring is so random that the PCM doesn't record them.


My spark plugs are very white, so I guess my old O2 sensors are uncalibrated. But still they work much better than the 2 aftermarket ones.


I can buy 2 brand new ones from DENSO but it's three times as expensive (180GBP each).

 

New O2 sensors are much more sensitive than used ones with many miles on them.

It's kind of normal to see the readings jump around a bit with new sensors.

If you're not getting any codes for the misfires, one has to wonder if they are actually misfires.

It's my go-to diagnosis these days, but maybe your cats are choked...

 

Cambo,
I have a question regarding the O2 sensors. Over the weekend, I found this in the Tech Guide regarding the downstream sensors:

With this in mind, it appears that all four sensors should also be replaced. Thoughts?

This thread is timely. I was about to order the upstream sensors for my Ford F350 and Jaguar. When I saw the above in the Tech Guide, I started to wonder if the Jaguar needs all four.

On a side note, the OEM for my truck is Bosch and I thought it was odd that Jaguar's OEM on the 2004 was Denso.

 

Hi Zazzy,
I have already read jaguar owners getting rid of random misfiring with replacing only downstream O2 sensors (which were giving a fault). It looks actually they fine tune the fuel trim because they have a Short Term Fuel Trim action too.
So my advice is to replace them four.
By the way it doesn't solve my issue...

 

Hello Cambo,
thanks.
OK, fresh O2 sensor will provide more fluctuations, I know. But have you seen in my chart the these new sensors are inducing a pumping of the STFT of more than 20%, and in reality it can go up to +/-15% (means 30% of variations in two seconds).
A pity that my OBD can't make a chart of the 02 sensor voltage, but I guess they provide big square curves between +0.5 and -0/5 steady for more than a second up and down.
I don't know what else to do than buying real DENSO ones at 3 times the cost.

 

Rockauto is selling the upstream (Denso) for $82 each. The Bosch are about $100. For my truck, the Bosch are only $37 each.

 

All four Densos ordered ran me about $300 last year, with the shop discount. Pretty good price, compared to list.

 

The upstream O2 sensor outputs are a fixed voltage, with a variable current as the signal. So if you want to see the raw signal you need to look at the current, not the voltage.

Yes the downstream sensors are also used for fuel management, but their contribution is relatively minor compared to the upstream sensors. In saying that some people have found in the past that changing the downstream sensors for new ones did solve engine running issues.

You did not mention which engine you have, or if there have been any modifications to the car, this sort of info is helpful to know.

There might be other issues at play here too, wiring problems or who knows what. The whole history of the car needs to be known.

White spark plugs sounds very lean...

You never know what you get with cheap aftermarket parts either...

There is no "calibration" needed for the O2 sensors, if they are the right ones (and good ones) they just work.

Sorry but sounds like there are possibly several issues at play here...

 

You can get good O2 sensors fairly cheap BUT an ad that lists front ones as the same as 4.0 or similar age engine is listing the REAR ones or is plain wrong. Fronts are different tech (current-based as stated by Cambo). Tend to cost a bit more.

You absolutely must fit the right kind (same for spark plugs).