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P0171 O2 sensor diagnosics - '97 XJ

It looks like I may have narrowed down the source of my problem. I pulled pin A24 from connector C1 and plugged the connector back into the PCM. No voltage was seen at either end of the BK/DG wire, but I was able to verify that the PCM's corresponding pin for A24 was reading 4.23v.

Before I start searching for a new PCM, can somebody positively verify that I should not be seeing ANY voltage generated from the PCM at this pin with key on, engine off?
 
Last edited:
I might have what you need.

Let me start by saying I had the same thing in my 98XJ.
I don't know anything about how it all works, but I saw a post on another forum just two days ago.

In the fuse box near the battery, there is a small unmarked 15 amp fuse.

On mine, it was in the group of the smallest fuses. It was on the back row, second from the passenger side. I replaced it & disconnected one battery terminal to reset the computer.

CEL went right off, & it runs fine now.

Hope this does it for you as well, and don't thank me if it does. Like I said this is someone else's posting.
 
SteveX82 said:
It looks like I may have narrowed down the source of my problem. I pulled pin A24 from connector C1 and plugged the connector back into the PCM. No voltage was seen at either end of the BK/DG wire, but I was able to verify that the PCM's corresponding pin for A24 was reading 4.23v.

Before I start searching for a new PCM, can somebody positively verify that I should not be seeing ANY voltage generated from the PCM at this pin with key on, engine off?
Click to expand...

Maybe you should perform all your same tests on the downstream sensor to see if it acts the same way. TN/BK wire, C1 pin A25.
 
Sidewinder CC said:
Maybe you should perform all your same tests on the downstream sensor to see if it acts the same way. TN/BK wire, C1 pin A25.
Click to expand...

It does not. I don't read any voltage at pin A25.

Also, it turns out that the fuse that Buckshot500 referred to was, in fact, burnt out. Unfortunately, replacing it didn't change anything.
 
Oops...I wasn't backprobing pin A25 properly. I am getting similar voltage to what I see on pin A24 (just over 4v). I've read a few unconfirmed statements about a 5v "pullup" voltage on the o2 signal wire that bleeds off as the heater warms up, but I don't know how trustworthy those statements are (see here: http://www.justanswer.com/questions/3896z-i-have-a-1998-jeep-wrangler-for-the-past-few-years-it-has).

At any rate, I pulled the new o2 sensor out of the exhaust to see what it looked like, and it was completely caked in carbon. I did the old propane / quench routine to clean it up, and reinstalled it into the exhaust. To my surprise, I fired the truck back up and the voltage began quickly dropping from over 4v down to the 0.500v range, and then began cycling back and forth! I excitedly ran inside to grab my wallet and cell phone for a test drive, but by the time I got back outside a few minutes later the truck was chugging again with the o2 voltage at a stagnant 0.150v.

I just can't win.
 
SteveX82 said:
Also, it turns out that the fuse that Buckshot500 referred to was, in fact, burnt out. Unfortunately, replacing it didn't change anything.
Click to expand...

I'm still trying to figure out which fuse this is that Buckshot500 described and you found blown on your truck. By his discription... "It was on the back row, second from the passenger side"...the PDC pic shows there's a 20A fuse, F20, in that slot, not a 15A fuse. F20 feeds the PCM, C3, C12 and the Ignition Coil. The fuse next to it (third from the passenger side) is 15A, F23, the Oxy Sensor heater power fuse. Are you sure it was the same fuse Buckshoot mentioned or was it fuse F23? Check it again. If it is blown again the sensor heaters aren't working....
 
Sidewinder CC said:
"It was on the back row, second from the passenger side"...the PDC pic shows there's a 20A fuse, F20, in that slot, not a 15A fuse.
Click to expand...

As you're looking at the fuse block (facing the rear of the truck with the passenger fender to your left), it looks like F23 fits buckshot's description. I checked it again yesterday and the new fuse is still good. Pulling the fuse with KOER eliminates the voltage going to the o2 sensor heater wire.

I noticed a tiny bit of an exhaust leak despite the new exhaust manifold, so I am going to pick up a new gasket to replace the crummy, ill-fitting chinese-made one that came with the header.
 
The info as I got it, was an unmarked 15 amp fuse in the box near the battery.
On my 1998 XJ (4.0L auto 4WD) it happened to be in the location I described.

It's possible that it's at another location in your box.
 
As an update, I installed a new Felpro manifold gasket this afternoon hoping that it'd cure my problems. There was a slight exhaust leak from the crummy gasket that came with the new header, so I figured the leak might be fooling the o2 sensor.

Unfortunately, I was apparently wrong. O2 signal was ~4v at startup before quickly dropping into the .3v-.8v range, where it oscillated back and forth for a few minutes. Great! By the time coolant temp reached 210º, signal voltage started dropping into the .1v range and the idle went to hell again. Damn....this is similar to what happened last time where the sensor tested well long enough to motivate me to take a test drive, but then went to hell as I was ready to head out on the road.

On a whim, I decided to exchange my brand new sensor since I had a warranty on it. Lo and behold, it seems to have done the trick (fingers crossed :explosion). I drove the truck about 20 miles this afternoon and pulled it back into the driveway expecting the worst, but I was happy to see the signal hopping back and forth between .3 and .8v.
 
Ok, I gotta ask:

Brand of the (suspect) faulty 02 sensor?
 
SteveX82 said:
On a whim, I decided to exchange my brand new sensor since I had a warranty on it. Lo and behold, it seems to have done the trick (fingers crossed :explosion). I drove the truck about 20 miles this afternoon and pulled it back into the driveway expecting the worst, but I was happy to see the signal hopping back and forth between .3 and .8v.
Click to expand...

Very good!

Did you change the rear one as well?

I changed both of mine the other day. I installed OEM sensors (actually NTK) and they made a very nice change in the idle performance and got rid of the hesitation during the warm up period drive-away I had been experiencing. 129,770 miles on my '99 XJ.

I'm with Joe, what brand sensor did you install?
 
Both sensors have been made by Bosch. Have they been known to be problematic?

And no, I only changed the upstream sensor. How expensive was your NTK? The Bosch set me back about $67 with tax.
 
I have read good and bad about Bosch sensors, but more good about NTK sensors. Search here and some other Jeep forums for O2 sensors.

You can get NTK sensors for your '97 XJ here:

http://www.discounto2sensor.com/sto...&Nr=AND(category:Exhaust,universal:0)&Ntt=ntk

I got Mopar sensors from the dealer for a few bucks more than Bosch, but I must admit, as a Chrysler retiree I get 25% off, plus the dealer I go to gives me and additional senior citizen 10% off. I found out that the Mopar sensors are made by NTK (marked on the side of the sensors) so all I paid for is the "Mopar" on the package and the Chrysler part number on the sensor. Next time I get them from Discount O2 Sensor...maybe...I don't know what they charge for shipping. Sometimes shipping charges are too exorbitant for me.
 
I always use Bosch, never had a bad one out of the box yet.
 
After driving the truck all weekend and putting on about 150miles, everything seems to be going great. I suspect that the "bad" new sensor might have actually been ruined in the brief few miles driven while the o2 heater fuse was blown. I am absolutely sure I measured +12v on the heater supply line early on in the troubleshooting process, but who knows....

Thanks again for everybody's help! Now I can make the trip out to Rausch Creek in a few weekends without my CEL staring me in the face.
 
JJacobs said:
Can you feel a single-cylinder misfire when it's doing this?
Click to expand...

What would this indicate?
I'm getting a CEL for my downstream 02 sensor, and sometimes when I get on the gas, I'll hear a single hollow 'ka-thunk' sound accompanied by a very brief acceleration stumble.
It feels like one cylinder misfiring.
 
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