I picked up a new oem exhaust system from a friend. At the time, I didn't know it was for a 01-02 till I opened the box and saw the threaded o2 sensors.

If I purchase a new o2 sensor(threaded) for my 99, would it be compatible? Or am I likely to throw codes.

Well, the bigger issue is not the threaded rear sensor but the front one. The '01-'02 are all Cali-type two cat systems. Is your '99 a Cali with 2 cats? The front sensor should be a flange. It also won't bolt up to the '99 49-state manifold at the top or anywhere in between. So I hope you are NOT a Federal/49-state one cat type 1999.

If your '99 is a Cali with two cats, I'd say you are good. I'm sort of an expert on here on these sensors, and honestly I've never seen this question asked. I do know that all the rear O2 sensors, Cali or Federal, from '96 to 2000, are exactly the same in electrical/sensor terms--they vary only in wire length. They only serve to monitor the performance of the cats; they have no effect on engine performance. They all have flanges.

I have never seen anything that suggests the emissions control system for the Cali-type '01-02 is any different from the Cali '99 and '00s other than the threaded connection instead of flange in the rear. But I've never seen it proven, either. You would be the first, as far as I know. (hopefully, there is someone out there who has tried this who will chime in.)

I do know the '02 threaded rear sensor will not plug up to the '99 harness--see pics below, '02 on top and '99 below:





So I suggest you get a universal type threaded O2 sensor Denso #234-4209 and marry that to your existing '99 sensor plug. I can't guarantee it will work, but it's your best shot. The wire colors WILL match up, black,black, white and blue.

https://www.amazon.com/gp/product/B0...k_ql_qh_dp_hza

EDIT: You know, now that I've looked at it a little more, I think they ARE the same plug, just the picture is taken in a different position. I can't guarantee it, but that's how it looks. So I'd get the '02 O2 sensor #234-4261:

https://www.amazon.com/Denso-234-426...denso+234-4261

If it does not plug up, you can still marry it to the '99 plug you should have. It's worth the extra bucks over the universal to avoid splicing in a very hostile environment, in my opinion. Please let us know what you do.

thank you for the response! i’ll have to look into it further, but will keep you posted.