View Full Version : downstream O2 sensor Question

10-17-2013, 07:10 PM
Ok ......I have read all the internet bullshit on what the downstream O2 sensor does and still do not know if you really need it. Some say it only throws a code for the cat.. others it affects engine fuel mixture and timing.
Can anyone that really knows the facts let me know before I waist money on one. 97 Cherokee 4.0 (downstream)
The faulty one shorted out intermittently, causing the cat fuse to blow and the engine ran like a bag of shit. Traced it down to a small rock dent on the sensor. I removed the little bugger and replaced the fuse and it seems to purr like a kitten. I need to air scare the jeep before I can re insure it and want to know before I waist money. ..HELP!!

10-17-2013, 08:12 PM
No engine codes should be good

10-17-2013, 09:20 PM
My brothers 96 cherokee was intermitently running shitty shortly after I changed the pre-cat o2 sensor. It had thrown a soft code for downstream o2 sensor. I asked around and some people were saying that it does affect air fuel mixture, while others said because the primary was so new and secondary was so old that that could cause the rough running, didn't make sense to me but I changed the downstream sensor and the problem went away.

But ya I've always thought the primary was there to adjust air fuel while the secondary was there to verify the cats doing its job...

10-17-2013, 09:34 PM
Wouldn't it be a open or closed loop system if it has 2 o2 sensors? Mandy's explorer has 4 and if one is fucked it won't complete the circuit this make the check engine light come on and run like a bag of shit,

10-20-2013, 12:55 PM
the first o2 sensor should be the "o2 sensor" that measures fuel mixture.
the one down stream of the cat should be the "catalyst monitor" which like the name states, keeps an eye on the efficiency of the cat.

both should have the ability of causing the MIL to come on. the latter of the 2 having a code about 'catalyst efficiency'

l would replace it.