: Where are my O2 Sensors?

01-03-04, 07:54 AM
I have a 1996 SLS with the NorthStar. It has 105K miles on it right now.

I would like to change the O2 Sensors as preventitive maintenance. I'll use the AC/DELCO of course, because this car didn't like the Bosch spark plugs, so I figured this thing is setup for OEM AC DELCO parts only.

If anyone has done this, is it an easy thing to do? I have always done this on my other GM cars and Ford Vehicles, and am familiar with those.


Tommy Smith

01-04-04, 03:40 PM
Hi Tommy. I saw your question with a big fat *zero* replies... and I couldn't help myself from spouting off at the keyboard. ;)

Your Northstar has four oxygen sensors. Two located ahead of both catalytic converters... and two behind each one also.

They are all somewhat easy to get to, and can be removed with a 7/8" open-end wrench. In some applications, a wrench is too clunky (big) to readily use, in which case an inexpensive oxygen sensor socket will work.

Make sure that the factory installed (on the new sensors) anti-sieze lubricant has not been smeared on the sensor cover (the slotted shell beneath the threads).

Do not over-tighten the new sensors. After the crush-gasket contacts the base, continue to tighten until the gasket is flat... and no more.

Make sure you hook-up a proper OBD2 diagnostic tool and clear any codes, then reset the fuel trim. If you don't reset the fuel trim, you will lose gas mileage and drivability.


01-04-04, 05:00 PM
2 cats? Uhhh no.

4 sensors? Yes. One on each exhaust manifold. One before and one after the catconverter.

Northstars are a bit different the most GM engines. Bank 1 is the rear engine manifold. Bank 2 is the front engine manifold.

So as the locations go....

Bank 1 sensor 1- rear exh manifold
Bank 1 sensor 2- pre cat o2 sensor
Bank 2 sensor 1- front exh manifold

Then the last one is mounted after the catcon. It measures the cat performance.

On OBDII cars. Stick with the factory O2 sensors. Bosch works fine on the older OBDI systems. Aftermarket ones can cause problems with OBDII systems...ie...false codes.


01-04-04, 05:57 PM
Correction noted and appreciated.

You aren't saying that there aren't Northstars that have twin cats from the factory... are you?


01-04-04, 07:19 PM
Factory Cadillacs have a single catalytic converter on them. Both front exhaust manifolds are connected via a Y pipe and then feed into the single catalytic converter. They only have a single exhaust pipe on them until about the rear wheels...then the system splits to the dual exhaust pipes.

The 'dual' rear exhaust outlets appear to be just that. But it all comes from a single pipe and converter.


01-05-04, 10:11 AM

Thanks for replying. I have read a few other posts by you, on EGR and carbon issues. Before I go to the expense of replacing O2 sensors, I think I will clean the EGR.

I did try to perform a top end cleaning, so maybe this procedure caused my excess carbon problem, and thus made the "SERVICE EMISSIONS SYSTEMS" light come one.

Now I just have to find how to reset the computer codes, and reset the fuel trip. I'll search around.

Thanks again.