Oxygen sensors on '97 Northstar

[hjb981]

There are oxygen sensors that control the fuel delivery and there are those that check catalytic converter function. I sometimes get the engine light turn on when it is cold, with codes that tell me that the heating is not working properly in the O2 sensors (PCM 0135 and 0155). Those codes are for the O2 sensors in the manifolds (one for each cylinder bank). I am now about to replace those two O2 sensors, and I have two questions:

What is the difference between AFS109 and AFS125? Both have the following (identical) description on Rockauto: SENSOR,HTD OXY(POSN 1) IN EXH MANIFOLD/CROSSOVER. They have different prices.

Is there any reason to replace the other two O2 sensors (one on each side of the catalytic converter)? Even if they were a bit "tired", that should still not effect engine performance in any way, correct? Only the two in the manifolds are involved in the closed-loop engine control, right?

[BodybyFisher]

Recently I worked on a 2002 Infiniti that had bad CATS. In researching and buying the CAT a visited a CAT manufacturer. He told me that the primary reason CATs go bad is 1) allowing an O2 sensor fault for a long time without fixing it as the engine never goes into closed loop and the air fuel mixture is rich damaging the CATS, and 2) he said O2 sensors get tired and slow to react and its good practice to replace them every few years.

That said, when I replaced the firewall O2 sensor on my 96 Deville, I bought mine from rockauto, when it arrived it had the wrong connector, ie male when it should have been female or visa versa. Watch out for the end connectors to make sure you get the correct connector. So much so, that I would purchase them locally. Take a look at your connectors before you buy them.

[Cadillac Jim]

Looking at the parts on RockAuto, the AFS109 lists the AFS125 as one of the parts that it replaces. On the other hand, other vendors list different parts for VIN Y and VIN 9 engines (270 hp and 295 hp, respectively, on the Deville and DTS). I would get a part from through a GM dealers, preferably locally as BodybyFisher suggests, and make sure that that you cross over to the oxygen sensor part number by VIN. As BodybyFisher says, it's best to have the old part in your hand for matching before you take it.

As far as the sensors on the CAT are concerned, they are much easier to get to, and can be changed in a few minutes from under the car. If one of them goes bad, you will get a code for either the oxygen sensor, the CAT, or both, but probably just the oxygen sensor.

[hjb981]

Thank you for the words of caution. Yes, rockauto replied to me by email and said the same thing: that AFS109 replaces AFS125, and that they just happened to not be out of stock of the 125 yet.

Has anyone changed the rear oxygen sensor (rear manifold sensor)? The FSM says that the engine frame should be supported and lowered no more than 3 inches. Is that necessary, or could it be done without lowering the frame?

[BodybyFisher]

I did mine with an 18 inch breaker bar, O2 sensor socket and long extension. Warm the engine to about 120 degrees. Use a universal adapter to get the correct angle on the O2 sensor if you have to, I have heard many having difficulty removing this O2 sensor, I didnt have a problem on my 96 Deville

[Cadillac Jim]

Be sure and keep the amount that you lower the engine to 3 inches, as the FSM says. If you lower it too much, you run the risk of kinking or breaking wiring harness connectors or brake lines. If you need more room, check around and disconnect things that are getting tight. Especially watch the EBTCM connector, the main wiring harness, and the brake lines to the ABS manifold.

[BodybyFisher]

For me it was unnecessary to drop the engine

[rockfangd]

I guess the firewall side one varies by vehicle. My seville was a nightmare. I ended up having the dealer change it because it could not be done without dropping the subframe. Unfortunately the replacement was defective.

But there was good news. the replacement sensor has a smaller body and you can get something in there easier. So I changed the defective one myself. I bought a special o2 socket from matco and used a long extension with my ratchet. I actually have pictures of the location. What a PITA though. It is tucked under the coolant pipe to the heatercore

[rockfangd]

here is a pic

[rockfangd]

and another closeup. Alll other sensors were cake. never had a problem since

[Cadillac Jim]

These are great pictures. What I want to know is, how did you get the *camera* between the engine and the firewall?

[hjb981]

I went here: http://parts-catalog.acdelco.com/catalog/catalog_search.php and searched for O2 sensors for my vehicle. It lists AFS109 for POS 1 (which I assume means the two in the manifolds) and AFS98 for POS 2 and 3 (before and after the cat. conv.). The same sensors are listed both for VIN Y and 9, which I think makes sense. I would have found it surprising if they would have used different O2 sensors depending on different valve timing of the cam axle (afaik that is all that differs between the two engines). Luckily I have an old sensor, so I can check that I get the correct ones without having to remove the existing ones.

Good camera work, rockfangd.

[rockfangd]

These are great pictures. What I want to know is, how did you get the *camera* between the engine and the firewall?

Thank you. it is a very good Kodak Camera, I have been taking thousands of pictures for a long time with it. even zoomed the pics are amazing