I have a '91 240 that I need to replace the catalytic converter. Currently the oxygen sensor is installed in the cat. I really doubt that I can get the sensor out so I was looking to replace it. Autozone lists several Bosch replacements but each of them states it is for use upstream of the cat. Other sites don't seem to mention this at all.

Is there a difference between the in-cat sensor and the upstream sensor?

 

If it's like my 740, the sensor is installed in the tapered part of the cat's body, just after the inlet pipe is welded on. This is "upstream", as it sits (just barely) before the "magic stuff" inside that does the work. A "downstream" is completely after the cat, so the computer can tell if the cat is actually doing its job.

 

These cars are way to earlier for 420.

 

So it sounds like the sensor from Autozone will work then.

Thanks for the reply

 

Might be a little late.... But doesn't the oxygen sensor just screw out? Or is it rusted in there With something expensive like that, I would find a way to get it out and use it till it dies.

 

Yeah - they just screw out. But this one has been in there probably 20 years. Doubt it will come out so easily. For now, I found that I don't have to replace the cat after all so it is staying in.

 

Hello,

I have a 1990 245 and I replaced the oxygen sensor in mine last year.

I was prepared for a battle to get it out, I went under the car ready to give it a PB Blaster bath. Just for grins, I decided to see how tight it really was. I put the sensor wrench on there gave it a pull and it was loose.

However, I have heard stories about how hard it was to remove, so just one guy's story here.

When you have to replace it, you may be lucky like me.

Good luck

 

I think this is the very first thing I changed on my 91'240.
Soaked it in PB blaster over night cut the wires of the end and used a 1/2" breaker bar with a socket. I pretty much had to hang on it to get it to turn even the slightest bit and it was pretty much the same all the way to the end of the threads. Not to say yours will be like that mine had about 239K on her at that time but mine was a bear to say the least!

 

If the O2 sensor tests good, why replace it? It does not matter how old the sensor is if it tests good and I highly doubt it's 20 years old...no offense. Yes O2 sensors can be a pain to get out but they are also expensive so I do not replace good O2 sensors if I do not have to no good mechanic will. So you are going need to do is 1. spray with a good lubricant and let sit 2. unplug sensor electrical connection at firewall 3. Slide wire through the closed end of your wrench until it is over the nut 4.try and get it loose 5.If that does not work take a piece of pipe or the handle from your floor jack(if you own one) and slide it over your wrench...try again. This should work because the pipe gives you incredible leverage. Try not strip the bolt while trying and do not force it. Take your time and it should be just fine. If it was installed correctly it probably will not be as hard as you imagine because there would have been some anti seize on the threads which keeps it from binding up. Also heat always helps too so you may want to let the car run a bit and get the cat nice and hot before you try or use a torch if you have one (just don't burn yourself trying..HA LOL!!) So when you do get it out and go to install it into the new cat make sure you put a fresh coat of anti-seize back on the threads before it goes in. Now if you are just hell bent on getting a new O2 sensor and you have the money to burn or your sensor is indeed bad, I recommend you get Bosch OEM replacement. Good luck!