Silly Check engine light o2 sensor

[Joliet Johnny]

My check engine light has come on so I went to Advanced to have them pull the code from it. It came back with a downdraft o2 sensor. The odd thing is that my XJ runs smoother and quieter than it ever has and gas mileage is about the same. I also noticed I can sit in traffic longer before the temp gauge creeps up. This is my 1st vehicle with 2 o2 sensors and in the state of IL we need to emissions test before we can renew plates every 2 years (eg. 1999 will be tested this year because its 2011 but 2012 I dont have to do it) I want to replace both sensors but Im unsure of which brands of o2 sensors to buy and the 1999 XJ uses the same sensor both before and after the cat right?

[Carves]

My understanding of the info in the FSM is that its the upstream sensor, that does the fuel mix fine tuning .... and the downstream one just provides comparison results against the programmed preset values ... so the PCM can "see" when the cat is inneffective and trigger the CEL.

What was the actual code they pulled ??

Bad connections in the wiring and a dodgy sensor are probably the main issues to consider.

NGK/NKN are rumoured to be original equipment ... RockAuto normally has 'em.

The different part numbers normally relate to the connector type & wire length .... not that one O2 sensor is different from another.

[cyclone70]

Wait at least a few days before replacing anything.
We have had the MIL light go on and go back out in a few
days because of crappy fuel.

[bluedragon436]

The O2 sensor before and after the cat are different part numbers.. I was wondering this when I had my CEL cut on.. then again I drove it that way for almost a year with the CEL on for the rear O2 sensor.. Apparently one is a heated O2 sensor and one isn't.. at least that is the way it was explained to me on why one is more expensive than the other.. I just picked up one out of the JY to make sure that my vehicle side of the harness wasn't messed up, and my CEL light went out instantly and haven't seen it since..

[prerunner1982]

The O2 sensor before and after the cat are different part numbers........Apparently one is a heated O2 sensor and one isn't.. at least that is the way it was explained to me on why one is more expensive than the other..

I was going to mention this as well, but my only experience with O2 sensors has been on a Chevy Blazer so I was not sure it that was the case with an XJ or not.

[dagr8tim]

I don't have a downstream sensor because the PO deleted the cat. I was considering looking at the wiring schematic for the heated o2 and seeing if I could rig up a simulator to get rid of my CEL.

[Mudderoy]

I don't have a downstream sensor because the PO deleted the cat. I was considering looking at the wiring schematic for the heated o2 and seeing if I could rig up a simulator to get rid of my CEL.

$100 for a CAT, $44 for a pipe from the header to the cat (with the O2 bung) and $65 for a bosch O2 sensor.

[dagr8tim]

$100 for a CAT, $44 for a pipe from the header to the cat (with the O2 bung) and $65 for a bosch O2 sensor.

I've got the cat from my 98 with 02. The PO just welded an adaptor to a chunk of straight tail pipe and put it in place of the cat.

[Mudderoy]

I've got the cat from my 98 with 02. The PO just welded an adaptor to a chunk of straight tail pipe and put it in place of the cat.

There's other ways to do it of course, I was just giving you worse case which to me sounded easier than trying to fab up something to fool the computer.

[dagr8tim]

There's other ways to do it of course, I was just giving you worse case which to me sounded easier than trying to fab up something to fool the computer.

I know. Tinkering with electronics is more fun. Althought I do not know if it would be illegal to build a simulator to make your computer think it had a sensor that was non existent.