: Working on mama's Caddy. Need help with O2 Sensor

10-13-09, 01:17 PM
As the title read, I'm working on my mother's 1996 Deville with a Northstar in it. She just had the engine replaced a few months ago because a repair couldn't be done twice due to block threads???? This car has had its share of engine but not as bad as some others in this group.

Anyway, I plugged in my old AutoXray and the only code it has is a P0139, O2 at Bank 1, Sensor 2 slow reaction. From what I've read here I might want to double-check the wiring first but I'm going to assume i have to replace it. My questions as simple to those of you guys that have wrenched on these before...

1. Where is the location of the O2 Bank 1, Sensor 2? Easy to get to?
2. Will a Bosch replacement do okay? Bosch is generally a good name.
3. What are the tools for the job? Will an open ended box do?

Replies are welcome and my mother would appreciate it.

10-13-09, 01:31 PM
You don't need a scanner: it's already built into the car. Read the sticky thread at the top of the main Northstar page titled "How to pull codes" - read the entire sticky because you're working with the transition year from OBD-I to OBD-II (OnBoard Diagnostics). When you get the codes, and whether they're Current or History, go back to the code link and decipher the P,C,B, and U codes.

OK....... this is for a later engine, but the sensor designations in the manifolds are the same.....Your B1 sensor 2 will be a before-cat sensor (one before, one after, unlike the third diagram) in the pipe just ahead of the cat - you need a slot-sided O2 sensor socket or a good crow's foot wrench. For the 2 cat sensors, easy removal, COLD. Don't get anything on the tip, and DO NOT twist or acutely bend the wiring. Bosch should be OK - some OEM Cadillac installations use Bosch sensors.

10-13-09, 03:26 PM
Bosch should be OK - some OEM Cadillac installations use Bosch sensors.
Really? I never new or heard of that. None the less, I'd stick with A/C Delco. Just my $0.02.

10-13-09, 03:59 PM
Yep - curious. Matter of fact, sitting in my spares box for the 2002 is a new 4-wire after-cat O2 sensor in a red-white-blue ACDelco box. The metal barrel is etched with the GM part number and the word DENSO.

GM sourced a lot of Bosch parts over the years - GM built the Opel, remember?

10-13-09, 04:22 PM
First off, thanks for the reply.

Your answers lead to a couple more questions.

I printed the third diagram and sketched in an O2 sensor before the CAT. I labelled it as B1S2. Is it safe to assume that the one shown in the diagram (after the CAT) is B1S3?

When you say it's an easy R&R, is that with the O2 socket? I guess what I'm asking is, should I buy an O2 socket. I don't have a crows-foot wrench, just combination (open and box) wrenches. It seems that if I can get at it with a socket, a wrench would work the same (locking 2 together if extra torque is needed).

As far as using the AutoXray scanner. I already own it and it's easy to pull and clear the codes, Plus it tells you what the error is w/o having to look it up.

10-13-09, 04:42 PM
Yep, the after-cat will show as B1S3.

Not sure the sensor connector will allow a box end wrench to fit past it, and you need the box/socket contact because those sensors can be a bitch to pull. Better to buy a sensor socket than burr the flats.........

Some have had good luck with allowing the pipe to cool to less than warm and shooting a bit of PB Blaster penetrant on the old sensor thread area, then let it sit for a half hour. But be careful - allow NOTHING to touch the tip of the new sensor.

10-13-09, 06:55 PM
..............BTW, if you have a proper sized box end wrench, grind/cut a slot out of the end so it will just fit over the wire. Like a flare nut tubing wrench.

10-13-09, 09:11 PM
If you're gonna do the job I'd get the socket. That's probably one of the few tools known to man that I don't have.

10-13-09, 09:43 PM
If you're gonna do the job I'd get the socket. That's probably one of the few tools known to man that I don't have.

Yeah - most box end wrenches are 12 point - O2 sockets are 6 point, like this, so less chance of rounding the threads and having an even bigger problem on your hands:



10-14-09, 11:24 PM
Picked up the sensor and a socket head that will take a 1/2" drive. (That's a knuckle buster!) Guy at the parts store said his computer called for 2 req'd. I'll try and put it in tomorrow night or Saturday and update the thread.

10-15-09, 05:26 PM
I think there are actually a total of 4 - 1 on each bank, the 1 before the cat, and finally 1 after the cat

10-15-09, 05:29 PM
That's true on your '97, but they deleted the pre CAT sensor about 2000 I think.

10-15-09, 10:57 PM
Okay, the deed is done but with surprises.

The first thing I noticed when I crawled under the car was that there was only 1 sensor in front of the cat. Then I saw that the sensor wires were sliced together. I took the new sensor out of the box and saw it had a connector on it already... but it was the same connector that I needed to plug into. I called the parts store and was told that the sensor both sensors near the cat were considered "after" cat sensors. Remember I didn't see any sensor physically after the cat, like in the diagram in previous posts.

I went back to the store where we dicussed further the Bank 1 - Sensor 2 location, which according to my scanner was the culprit, and was told that Bank 1 sensors are near the cat and Bank 2 sensors are in the manifolds and I had a manifold sensor. I got the "after" cat sensor that goes physically just before the cat and the connector was correct.

I installed that sensor without incident and will wait my mother's report. It takes a couple days unitl the P139 code comes back after a clearing.

While under the car I again looked for the real post-cat sensor and saw nothing. The parts store guy said that it's on top of the tube where it must be replaced by using an access door under a seat. I just didn't see anything there.

This is a bit strange to me. What say you caddy experts?

10-16-09, 11:30 AM
Go back to Post #2 for the names of the sensors (at the bottom corner of each line drawing - click on it to enlarge........).

IF you had 4 sensors the cat sensors would be B1S2 and B1S3.

The front bank (near the radiator) is B2, the rear bank is B1, so the O2 sensor numbers follow in logical order.

I have never heard of an access door under a seat. The after-cat drawing is of a 2002 STS installation and was included for clarification. Even Cadillac would not put an O2 sensor on top of a pipe.

The spliced wire is the culprit: O2 wires are NOT to be cut, spliced, soldered, or twisted. Is the cat original or aftermarket ??

10-16-09, 11:34 AM
This guy obviously does not know what he is talking about. Bank 1 & 2 is referring to the right and left bank of cylinders, not "Bank 1 sensors are near the cat and Bank 2 sensors are in the manifolds". Bank 1 is the right or firewall bank. Bank 2 is the left or grill side bank. B1S1 is the manifold sensor. B1S2 will be the pre CAT sensor. The '96 should have a B1S3 aft CAT sensor. There is no access door under the seat.

By the way, describe "spliced together". These wires should NEVER be soldered. I think there is a special kit and procedure to repair them id need be. Ewill once discussed it.

11-06-09, 11:39 AM
I am getting the same code on the same car. So you had to get the after cat part even though the sensor was before? Was bosch part #13444 the one you ended up using?

11-06-09, 10:06 PM
3. What are the tools for the job? Will an open ended box do?

Think about this wrench.......Is it from one of those goofy sites that has muffler bearings and such? LOL

It's either an open end or a box or a combination. gave me the best chuckle I've had all day!:thumbsup::thumbsup::thumbsup: or :stirpot::stirpot::stirpot:

11-08-09, 08:04 PM
O2 Sensor socket.

Don't use a Bosch sensor. Stick with A/C Delco.