exhaust issue-need advise...

[Cadillakin'98]

I work in a body shop but have a very good understanding of the mechanics of automobiles as well. But I need some help from some technicians (mechanics will also do I suppose) regarding exhaust. Heres the deal:

SES light came on my sisters '96 Blazer, Twilight Zone said code is for O2 sensor in bank 2 sens 2 (I think). Light went off next day but I still replaced sensor and took vehicle to get inspected. It passed. 2 weeks later my sister says her light came on again. (By the way I did reset the computer after replacing the sensor) Twilight Zone says it reads 2 codes, both leading to O2 sensors, this time in bank 1-sens 1, and bank 1-sens 2 (I think). Well, drove it to go buy o2 sensors elsewhere (hate Auto Zone) and noticed it runs a little rough-possibly due to bad O2 sensors. Decided to by the remaining 3 not yet replaced on vehicle although computer only showed 2 more. But, this is were I need expert advise.... is the problem simply the O2 sensors, or is there another cause? Perhaps the catalytic converter?? Why would 3 O2 sensors go bad? From what I understand they rarely go bad like that. I do know the code only indicates the O2 sensors sense a problem that the computer can't compensate for to read right, thus meaning the O2 sensors are okay and there could be something else wrong. What should I test and how to locate the "true" problem? Oh yeah, it has been a week since I bought the O2 sensors (not yet installed) and sister said light came on again. Don't know what the code is but thinking its the last O2 sensor, thats why I wanna make sure I am correcting the problem, not just what the Twilight Zone thinks is the problem. Any advise is greatly appreciated

[BlazerLT]

Ok, let's get your O2 sensors straight.

Bank 1 Sensor 1 = Drivers side exhaust O2 sensor
Bank 2 sensor 1 = Passenger side exhaust O2 sensor

Bank 1 sensor 2 and bank 2 sensor 2 are the same O2 sensor and that is the sensor infront of the cat convertor.

Bank 1 sensor 3 and bank 2 sensor 3 are the same sensor and it is the O2 sensor after the cat or which is known as the cat efficiency monitor.

1) The first sensor you replaced, was it the O2 sensor before the cat?

Also note, dirty MAF sensors will also cause these problems and the random ghost O2 sensor problem seeing they are related to each other through the fuel management system.

Remove the MAF from the truck and spary throttle body or carb cleaner through the MAF sensor and dissolve the black crud on the two sensor wires.

You might have to use a toothbrush to GENTLY agitate the crud off while wet with the cleaner. Then rinse and reinstall while resetting the computer by pulling the negative battery terminal from the battery and removing the two fuses for the ECM in the fuse panel.

Other than that, when was the last time this truck was tuned up?

What temperature does the temp gauge get up to after the engine has been running for a while?

[tom3]

Wouldn't hurt to remove and clean the EGR valve seat. Just a bit of carbon will cause all sorts of problems. Must seat really good and the electronic valve is known for this.

[Cadillakin'98]

I thought about the MAS today @ work and think it definitaly wouldn't hurt either way. This is my sisters vehicle as i mentioned and unsure what the tmp gets @ when warmed up and driven a bit. My sister is never around long enough for me to take a good look @ it, and of course never has time to answer any of my ?'s about it. She simply tells me a light is on or it runs funny, that is once she remembers to.
Anyways, I appreciate the guidance and suggestions. Plan on messin w/ it this weekend come hell or high water, been putting it off for 2 weeks, due to doubts of all sensors really being the true issue. Well, hopefully ya'll can answer any ?'s for me around the weekend if needed. Thanks again.