I have not personally witnessed the random O2 sensor failure causing the check engine light to come on. This past weekend my wife was driving my 93 Blazer on about a 40 mile trip and told me the light was coming on at random times going down the highway. She said it did it again on the way back 3 days later. I only drive it to work going about 8 miles one way and have never seen the light. I scanned the ECM and while I forget the code it was an O2 Sensor failure. The ECM light is not on, but would come on and go off, causing the ECM to store the code.

I observed the mVolts for the O2 sensor with the system in Open loop and it was running between .4 and .7 volts, when I generally recall it used to run between .1 and .9 volts, constantly adjusting up and down.

I also noticed the fuel system was pretty much staying on the "Rich" side, rather than flopping between "Rich" and "Lean".

I also checked the EGR Duty Cycle. It used to run around 40+% running down the highway. I checked it in all kinds of condition and it reads 0% constantly with an occasional bleep of 60+% for less than a second and very random, during constant speed, slight acceleration and slight deceleration. I'd say 99% of the time it is reading 0% Duty Cycle.

Is my EGR in need of replacement? I would think it would throw the light and code if there was a problem, but I really don't know. Is the O2 sensor going bad and causing the EGR to not respond?

 

Well, after much deliberation, I replaced the O2 sensor. I watch the readings for a few days and decided that the existing O2 sensor wasn't responding any where as crisp and quickly as it had when I first bought my scanner. Originally, I was getting readings for the O2 sensor mVolts ranging from 150 to 960. It was very methodical about the process as well. Lately, the O2 Sensor mVolts had been hovering around 600 to 750. My "Rich" flag was on about 80% of the time and my cross counts were in the teens or lower, always.

I mentioned previously that my EGR Duty Cycle was all but none existent and wondered if my EGR valve was going bad (without throwing a code) or if the O2 sensor could manipulate the EGR Duty Cycle. Turns out the O2 Sensor replacement was a cure all for what ailed my Blazer! My cross counts are non stop now and the mVolts for the O2 sensor are going from 50 to 990+, constantly! The Rich/Lean flag is tripping at probably less than a half second interval. This Blazer is running fantastic now! All my low end bog and hesitation is completely non existent now and my exhaust smells like a clean running exhaust instead of real foul odor.

I bought an AC Delco replacement O2 sensor. It replaced a Bosch, which I have no idea how long it has been in, but since Swartlik is always preaching that Bosch O2 sensors and these Blazers don't get along I figure the 30 dollar difference was well worth it (cost me 70 for the AC), and since the O2 sensor fixed what ailed my machine, I couldn't be happier.

BTW, my EGR Duty Cycle is constantly active now anywhere between off idle and belwo full throttle, generally in the 30 to 60% range. I didn't replace my EGR valve.

 

Sorry... I must have missed this when you posted the first time. Glad you got it figured out. Scan tools are a necessary evil with the computer controls we have now and once you have one, these vehicles aren't really that difficult to trouble shoot. My new scan interface has bi-directional controls which will allow me to temporarily command values for the EGR valve, IAC valve, and other sensors/actuators that can be manipulated. You can then see how other sensors and the PCM react to what you have commanded to trouble shoot even further. It basically is a Tech2 scan tool without all of the cost, but requires a laptop to use. I'll be writing a review of it once I receive it (tomorrow per UPS.com) and get a chance to use it.

I am also glad that you decided to go with the AC Delco. The Bosch sensor will act similar to the AC Delco in the beginning, but they fall off rather quickly (some more quickly than others) when compared to the life of an AC Delco and the "Universal" sensors are absolutely horrible. I think I have scans somewhere of a brand new Bosch on one side of an engine (1997 Caddy Northstar) and a brand new AC Delco on the other side. The Delco was switching between rich/lean over twice as fast as the Bosch and both were brand new out of the box. Then my whole experience with Borg Warner O2 sensors on my Bravada with one failing after a month of use (less than 500 miles). My AC Delco's have been in for the last 6 months and over 3000 miles worth of use without issue and the scan I took last weekend is identical to the one I took when they were new.

Anyway, enough rambling. Glad you got 'er fixed!