I'm replacing the Pre Cat O2 Sensor in my 1999 Monte Carlo Z34. I can see it from the top of the engine compartment but it looks like a nightmare to access with any ease. I know I'll have to jack it up and remove it from the bottom of the car. I plan on disconnecting the negative battery terminal before I start, and cutting the old wire off the sensor to try and use a sparkplug socket to remove it. I sprayed the sensor with PB Blaster and let it soak for 3 day. And I'm going to run the car for a few minutes before I start to heat it up, because I've heard it's VERY difficult to loosen. Does anyone have any feedback for this repair. I am not sure what I need to remove from the bottom of the car to access the sensor freely. Thanks in advance.

 

You're not gonna be able to get to it from under the car, especially if you're talking about the one that's on the back exhaust manifold.

It sounds like you have the right idea, but here's my suggestion: unbolt the dogbones and rock the engine forward to give you some more room to work back there. I think I ended up using an adjustable wrench to get mine off when I changed it. If worse comes to worse, I'd say remove the rear exhaust manifold and take out the O2 sensor with the manifold off the motor.

Good luck.

 

Personally, I wouldn't cut the wiring. I'd go to the local Autozone or O'Reilly and borrow an O2 socket. If it's the OE sensor you may need to use a breaker bar to crack it loose. Chase the threads in the bung (so you don't accidently cross-thread the new sensor) and apply some anti-seize to the threads of the new sensor if it isn't already on them. Use an AC Delco or Delphi sensor. People have reported having problems with other aftermarket brand sensors.

 

I have a complete write-up when I did mine and can't find it...

:edit:

Here's something I wrote on another forum of what I did:

Run the car for a few minutes to get the rear exhaust manifold warm. (helps loosen the o2 sensor)

I rocked my engine forward to get more room.

Unplug the negative terminal on the battery.

Disconnect the plug for the o2 sensor from the wiring harness, pull out the christmas tree plug from the bracket it is mounted to.

Using a high quality O2 sensor socket (since cheap ones can bend) connect to the O2 sensor, using a few socket extensions and a wrench to make a cheater bar prepare to give it some muscle.

When you break it free, follow with a thread chaser to clean it all up so the new one seats well.

Install and torque the new o2 sensor to 35 (36?) ft/lbs.

Plug the new o2 sensor in to the harness, and clip the new christmas tree to the bracket.

Reinstall negative battery terminal (or leave unplugged for a while to reset the PCM to learn readings from the new o2 sensor)

 

I really appreciate all the input from everyone so far. I'm pretty sure that between the combined advice, I should be able to tackle this repair now. I'll keep everyone updated on my progress. Any special precautions I should be taking whle removing the dogbones to rock the engine foward ? I'm pretty determined to fix this. The car runs awful, especially while in idle and I hope this fixes the issue.

 

Nope, just take out the bolts and pull the engine toward you, there's still an engine mount and a tranny mount holding it in place.

 

I found removing the STB to be quick-n-easy and provided adaquate room to get my breaker bar positioned over the sensor.

 

If you don't want to PULL, you can rock the car forward and back and you'll see the engine moving. When you see it moving forward, step on the parking brake and it will stay there haha