Replacing an Oxygen Sensor easy as pie!
 

The OBD Code for 4-6 pre CAT oxygen sensor has been triggering the dash light about once a year for the last few years on my 1996 993. This indicated a slow to warm up but not dead sensor so I saw no need to replace it. Usually it comes on when the weather becomes cooler in the fall. This year I got two codes one for the sensor and one for the sensor heater that when cleared came back in a few days. So I changed the sensor. To my surprise all four oxygen sensors are easily accessible from the rear under the car just behind the bumper. To change the before CAT 4-6 sensor the hardest part was screwing the clamp in that holds the sensor wire clear of the exhaust system as it runs to its electrical connection point.
The only tools needed are an adjustable Crescent wrench to unscrew the sensor a 10mm socket, long extension and ratchet to remove and replace the wire clamp and a small mirror to see what you are doing under the car. If you put the car on jacks so you can get your head under the car the time to do the change out drops from 20 min. to probably less than ten. Unlike on my Volkswagen where I had to apply heat to release the sensor threads I used a little penetrating oil on the sensor. In retrospect this may have not even been necessary.
Very easy job – a 2 on a 1 to 10 scale of difficulty
Andy .

Easy DIY's are the best. Cheers.
What symptoms did you notice before changing them Andy?
And did you notice any difference after changing?

I know there are two type of oxy sensors - wide and narrow. Did you change to OEM narrow band?
Any one ever change to wide band and did it make a difference?

TIA

Ken

To add one more question, are O2 sensors expensive?

I only changed the one that the OBDC said was not functioning. I replaced it with the same type. I believe each vehicle uses the type it was originally designed for. I purchased the sensor a long time ago for about $60. I think the wide vs narrow band relates to which one your car was designed to use. Symptom was that the light on the dash came back shortly after the faults were cleared. No driveability issues.
Andy

Glad to hear that the proceedure is simple! Going to be tackling this on the weekend.

They can be if you order them from the dealership. I did some poking around
and suprise, suprise FDM to the rescue with the best price around! I just
ordered a set of 4, to replace all of mine ~ even though only one is
causing issues.

Follow up years later
 

It is now 2017 my car has 100k miles on the odometer and it is many years later than when I replaced the first one above.

I ran the codes and the heater is dead on the driver side LHD(bank 1) before CAT sensor. I also have an air injector code.

I suspect the ignition computer uses the oxygen sensors to determine if oxygen from the air injector is influencing immissions for the few moments it is running at start-up. If the sensor's heater isn't working the computer gets bad information as the sensor does not warm up into its operating range quickly enough.

I also have a code for a slow sensor after CAT on the passenger side LHD (bank2). This is not of much a concern but the sensor is inexpensive and I will be going to the trouble to put the car on jack stands anyway so I will replace it as well.

Andy

Thx
 

Thanks for taking time to update your earlier post. Very thoughtful and helpful.