How do you test an O2 sensor
 

I have a bad O2 sensor in my Passat and I think I have one in my 944 as well. I was wondering if there was a procedure for checking them??? The Passat has more than one so I don't feel like replacing a good one but the engine is throwing a code that one is not working right. The 944 is in the exhaust but the exhaust is off the car. It is also stuck pretty good. Any tips on getting it out??????

PB Blaster, a mallet, and a 22mm wrench. You can check it with the engine running - the heating element will get it hot, and the output wire will send between 0.1 and 1V DC

Another way to test is to check the exhaust gas with a gas analyzer - you have to check the gas before the cat, but after the O2 sensor (that is what that little tube right after the O2 sensor is for)

If your tube is gine (most are) and the hole is capped, you are pretty much limited to the first method - or just replacing it.

AFJuvat

I will just replace it while it is out. I have had the car for 6+ years and 60 000KM and it is running rich so I will just get it out will the exhaust is out. Bosh generic will work right? just need to crimp on my old ends???

try soaking the O2 sensor in B12 (carbcleaner) over night, and then blow it out real good with compressed air. In the past when I had a customer complain on the high price of an O2 sensor and didnt want to buy one, I suggested this and got some good feedback from it (about 50/50)! Worth a try for the sake of $150.