Loose rear oxygen sensor (solved a rattling problem, but...)

I'm a new Tacoma owner and new to this forum, so first of all, hello! This forum has already been very helpful. Soon after I bought my truck (2001 Tacoma 4x4), I noticed a rattling noise coming from the undercarriage in low gear. I was afraid it was my transmission, but after checking out some threads here, I gave the exhaust a thorough going over and discovered that the rear oxygen sensor was loose. It was rattling around against the exhaust pipe. I tied it up with some wire for the time being.

However, I'm concerned about why it was rattling: the gasket seems to have completely disintegrated, and the nuts originally holding it on have corroded away. All that's left of them are some nubs at the bottom of the bolts.

What puzzles me is that the check engine light hasn't come on, even though the sensor was not sealed onto the exhaust. Can the sensor really still be working properly, even if it's been rattling around against the exhaust without a gasket?

I've looked up the parts to replace the sensor, but I wonder whether it might just need a new gasket and nuts. What are your thoughts? Anyone had a similar problem?

 

Yes this is plausible. The rear o2 won't throw a code immediately. The rear o2 simply oscillate between 0 and 1v and the ECU just makes sure it's oscillating, which if it has any contact with the exhaust stream, it would be. I have my exhaust cutout open all night at the race track and I don't get a code for rear o2.

 

Thanks, guys! That's really helpful. So the sensor may actually still be working after all.

I think I may go ahead and replace the sensor anyway, rather than just get a new gasket. Do you guys know what size nuts to get for the studs? Most of them seem to be 18m/1.5 - is that right? I can't find the part no. for them.

 

8mm x 1.25

 

Many thanks!

 

For a follow up to this problem and a full description of the fix, see this thread.