Moderator Help for Downstream O2 Sensor Sticky
 

Could you please edit the S2 sticky to read "For everybody except Dave51's XJ"?

https://www.cherokeeforum.com/f2/wha...answer-244489/

According to the FSM, mine DOES participate in adjusting the air-fuel ratios.

TIA.

BTW, although I have a California Emssions, the above applies to non-California as well:

Seems like this started in 2000. The 1999 FSM says:

The downstream heated oxygen sensor is located
near the outlet end of the catalytic convertor (Fig. 6).
The downstream heated oxygen sensor input is used
to detect catalytic convertor deterioration. As the convertor
deteriorates, the input from the downstream
sensor begins to match the upstream sensor input
except for a slight time delay. By comparing the
downstream heated oxygen sensor input to the input
from the upstream sensor, the PCM calculates catalytic
convertor efficiency.

There's a lot of debate, opinions, and "I know somebody" out there that claim the downstream sensor participated in helping determine air-fuel mixture since the beginning of OBDII.

Sample:

https://www.jeepforum.com/forum/f13/...ensor-1124945/

At any rate, it's easy enough to find out. Get an adjustable DC low voltage generator. Run 0.2, 0.6, and 0.8 volts respectively for the S2 signal. If the fuel trims change, that would confirm it has role.

IMO it started in 1999. Although the factory parts manual for 1997-1999 does not show a



https://cimg3.ibsrv.net/gimg/www.che...fd152e4796.jpg
catalytic converter, aftermarket manufacturers sell one, i.e. MagnaFlow Direct Fit Federal Catalytic Converter 24997 (this looks to be exclusive for 1999) so it would seem that there are at least some 99's with tricats.

In 1999, Chrysler applied for a patent https://patents.google.com/patent/US6256981 Fuel control system with multiple oxygen sensors. In it a system composed of 2 catalytic converters and 3 oxygen sensors is discussed:

I believe this is where the downstream oxygen sensor began taking on the added role of monitoring air-ratio (although in practice, the first catalytic converter turned into 2, and the third oxygen sensor never materialized).

Interesting..FSM says one thing, another dudes swears its wrong, next dude swears its right

I was gonna nominate you to be the man to end the debate,,but you cant, having a later version

maybe I can..I just replaced the Cat and rear O2 on my '96, and have live data...wouldnt unplugging the rear O2 affect the Fuel Trim ? (if in fact it does)

(If the Short Term Fuel Trim does not change, that would pretty well answer the question ?)

I suspect the FSM is correct, and it does not, as this is purely a anti-pollution measure, not performance (as I understand it)

Comment, when I got this XJ, the rear O2 wires were cut, No CEL, but a P0138 code stored, wouldnt clear

Cat started rattling enough to piss me off, so I got another, and a rear O2

apart from sounding a lot better, I do feel it seems to be slightly more responsive, but not using less fuel

Mine is an early 1999, made in 11/98, and it has just one cat, and two O2 sensors. I know when the downstream sensor is unplugged it throws a code, but performance seems unaffected. I did not check the trims with the sensor unplugged.

My 96 fuel trims seem un affected by the downstream o2 sensor. I run without a cat and a spacer. No code is thrown for years with this set up and the I/M monitor is in the ready state. If I ever get around to it I will hook up my second o2 sensor output on the FIC to the downstream and offset voltage.

I think the key is having tricats.

Would love to hear if anybody with a 99 has a tricat, and if so, it also has the elusive (or mythical) 99.5 0331 head.

The 2000 Factory Service Manual has odd wording if indeed they are trying to convey the fact that the down stream oxygen sensors are used to control mixture. It is a stretch to think a sensor whos input is altered by a catalyst is used to control mixture, while maintaining good drivability.
I suspect the manual has an error, would not be the first time a manual was in error. The wording is hideous, vague.

Well the evidence points quite to the contrary. Sullivan et al submitted the application for the patent in 1999 for the use of multiple oxygen sensors, including post-cat, to determine air-fuel mixture:

https://cimg6.ibsrv.net/gimg/www.che...35d078a5ec.jpg

Yet the manual still does not say the down stream sensors control the engine. The info provided that the down stream sensor they allow the ecu to "learn". This learning sets targets for the upstream sensor. Thus the final control is the upstream sensor. down stream sensors create a learned map by which the ecu uses to control the fuel so as to get the desired result from the upstream sensor.

Well, I hate to resort to the "Does so!" style of debate, but "Does so!" Again, the manual specifically states:
The upstream sensor is a tool, not a controller.

Referring back to the patent (and admittedly, what exactly ends up in the ECU is unknown, and there's a LOT of stuff going on):
which makes this algorithm primarily emissions-oriented.