I have a '99 SE and I've run into a little problem. My check engine light recently can on so I scanned the code. It's telling me that it's my secondary O2 sensor. I'm pretty sure that the one in the cat. Just checkking before I go ripping it out. Any help is good help. Thanks

 

If i recall correctly the sensor AFTER the cat is there for one reason.
To tell you that you need a new cat.

The one (ones) BEFORE the cat help control fuel/ air mixtures.

 

Aw crap hopefully that's not the case guess I'll find out thanks

 

Your cat may be fine.......the downstream O2 sensor may be bad or the heat element in the sensor may be bad.

http://www.aa1car.com/library/o2sensor.htm

 

Thanks! That's a pretty good write-up. I'll check out the sensor tomorrow when I've got a little more light.

 

Yea, I got my Jeep without a downstream O2 sensor and a check engine light. I put an O2 sensor in and it went off and gas mileage increased because it wasn't running in open loop anymore. This lasted till the other day when it got torn off during a hill climb. No big deal, I'll get another one from the junkyard and call it a day.

 

Probably a good idea considering I priced out a new one and the parts guy I always go to wants $100 for it.

 

That's a rip off. I got mine for free. And I'll get another one for free, haha. It's always good to be cool with the junkyard guys. I'll just tell him, "Hey, you gave me a crappy O2 sensor, it couldn't even handle a little offroading!" then he'll reply with, "Well how much did I charge you?" and when I say nothing, he'll just laugh and I get another one for "Free.99"

 

What yard do you go to? I'm always over at King Auto in Dayton, but lately the haven't gotten any XJs

 

H & H down the end of Texas Road. There are 3 or 4 Cherokees right now and a couple of Grand Cherokees (which most parts are interchangeable with). I don't go anywhere else, but there, and I'm there at least once a week.

 

Oh ok cool I guess I've never made it down that far texas road lol I'm up and down there a lot thanks

 

It's pretty far. You cross Route 9 and keep going for a little bit. Go through a 4 way stop sign, pass a big blue plow on your left side and it's right there. There are two yards when you pull in. The first one which has the scale for junking stuff, and then the one after that. Don't go to the one with the scale because it is not really part of H & H and I don't know what that guy charges for stuff.

 

The downstream O2 doesn't determine open or closed loop or air/fuel ratios for that matter......that is the job of the upstream O2. The downstream O2 monitors cat efficiency. We are temporarily without a downstream O2, our CEL is on, ScanGauge displays code P0138 (Bank 1, Sensor 2) however, the ScanGauge also shows the system still goes into closed loop within a few seconds after engine start-up, just like it did with the downstream O2 installed. In fact, our son drove it back to school today (447 miles) and averaged 22.1 mpg going 70-80 mph. As a side note, a 195F t-stat is not necessary for good mpg as we changed our t-stat to a 180F and all seems well, including the heater.

 

"When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture.If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions"

"A downstream oxygen sensor in or behind the catalytic converter works exactly the same as an upstream O2 sensor in the exhaust manifold. The sensor produces a voltage that changes when the amount of unburned oxygen in the exhaust changes. If the O2 sensor is a traditional zirconia type sensor, the voltage output drops to about 0.2 volts when the fuel mixture is lean (more oxygen in the exhaust). When the fuel mixture is rich (less oxygen in the exhaust), the sensor's output jumps up to a high of about 0.9 volts. The high or low voltage signal tells the PCM the fuel mixture is rich or lean."

That's all from the link you posted earlier and explains my gains in gas mileage and the whole open-loop problem.

 

oh ok got it