EGR Duty Cycle and O2 Sensor Failure

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?

This sounds like a O2 sensor problem, or a short in the wiring to the O2 sensor. It could also be a ECM problem too, check the different sensors and see what their readings are from a scanner. That way your getting readings of what the ECM is interpreting them as.

The EGR is controlled by the parameters that are present when the truck is running. So if the truck is running rich, the ECM will adjust the EGR according to the conditions. If the truck is not running right, or if the ECM is not good, it will not be controlling the EGR system correctly.

It could also be a ECM problem too, check the different sensors and see what their readings are from a scanner.

What sensors should I be looking at? What would normal readings be for a fully warmed up engine on the highway with the cruise on at say 65 MPH. All of my information (Duty Cycle, mVolts, Rich) I previously posted was from my scanner. The 93 only has the one O2 Sensor in front of the Cat.

How many miles on it?

If you have over 60,000miles, it is due for a change. You are just seeing the start of the failure.

How many miles on it?

If you have over 60,000miles, it is due for a change. You are just seeing the start of the failure.

This vehicle has 116K on it. I believe the EGR has either been cleaned or replaced before, have no idea about the O2 sensor. When I replaced the injector, I checked the EGR valve and it had the gasket with the wire screen already. The EGR valve looked good and clump free, along with the intake. But other than that I would have no idea how long hte EGR valve has been on. You might be right here.

This vehicle has 116K on it. I believe the EGR has either been cleaned or replaced before, have no idea about the O2 sensor. When I replaced the injector, I checked the EGR valve and it had the gasket with the wire screen already. The EGR valve looked good and clump free, along with the intake. But other than that I would have no idea how long hte EGR valve has been on. You might be right here.

I am only talking about the O2 sensor here.

If it has that many miles and has gone through a CPI/Nut kit failure, its' lifespan will be challenged.

I would suggest a O2 sensor change in the near future which will stop and rich conditions and to get your engine running as clean as possible.

Mine were replaced at around 120,000miles and although they were not failing to the point of throwing a code, I did see an immediate improvement in engine performance and fuel economy when they were changed.

Just nremember to run the engine for 5-10 minutes before the change to aid in removing the old sensor. Don't attempt it with a cold exhaust system or you will be facing some seriously extraction problems. Use an O2 sensor socket for the removal and use a rubber mallet for force to remove it. Also soaking it in PB Blaster for 10-15 before removal wouldn't hurt either. Just make sure you are turning the rachet in the right direction. You don't want to make this job harder by tightening it more.

I'll swap out the O2 sensor first then. I've had it out before and reinstalled using a light coat of anti-seize. The only difficult part of the O2 sensor change on this vehicle is the wire pig tail runs up the fire wall in a real tight space and the harness is held in place with a factory plastic clip (making me think it is original).

Would you recommend an AC Delco or will one from a parts store suffice?

........If it has that many miles and has gone through a CPI/Nut kit failure, its' lifespan will be challenged............

I had about 100K miles on my 93 S-10 Blazer when the CPI and/or Nut kit failed. I had EGR codes set but never an O2 code. In closed loop, the lamp stayed on continuously indicating that the engine was running RICH all the time.

After replacing the CPI and Nut kit, the lamp had an equal ON/OFF cycle of about 1 second each in closed loop. I didn't replace the O2 sensor because it seemed to be doing its job of controlling the fuel mixture. Until I get an O2 code set, I'll probably leave it alone because the engine seems to be running smoothly and responds correctly for various load conditions.

I'll swap out the O2 sensor first then. I've had it out before and reinstalled using a light coat of anti-seize. The only difficult part of the O2 sensor change on this vehicle is the wire pig tail runs up the fire wall in a real tight space and the harness is held in place with a factory plastic clip (making me think it is original).

Would you recommend an AC Delco or will one from a parts store suffice?

Nah, go with Bosch, much cheaper and they were the company that actually designed the GM O2 sensor system.

I have used Bosch for my swap and not one problem.

They come with antiseize already so don't add anymore.

I had about 100K miles on my 93 S-10 Blazer when the CPI and/or Nut kit failed. I had EGR codes set but never an O2 code. In closed loop, the lamp stayed on continuously indicating that the engine was running RICH all the time.

After replacing the CPI and Nut kit, the lamp had an equal ON/OFF cycle of about 1 second each in closed loop. I didn't replace the O2 sensor because it seemed to be doing its job of controlling the fuel mixture. Until I get an O2 code set, I'll probably leave it alone because the engine seems to be running smoothly and responds correctly for various load conditions.

Don't assume that if it is running smoothly that it is running as efficiently as possible.

Normal O2 sensors will only work properly until 60k or a little more before they will slowly degrade.

You having 100k and a lot of those miles being under really rich conditions is the ideal pointer that you should be swapping it out.

You have to think about preventative maintanence rather that what is happening now. It is like people only replacing there plugs when the plugs are so bad that the engine won't even run. You wouldn't do that so why do that to the most important sensor in your fuel and emmissions system.

Don't assume that if it is running smoothly that it is running as efficiently as possible.

Normal O2 sensors will only work properly until 60k or a little more before they will slowly degrade.

You having 100k and a lot of those miles being under really rich conditions is the ideal pointer that you should be swapping it out.

You have to think about preventative maintanence rather that what is happening now. It is like people only replacing there plugs when the plugs are so bad that the engine won't even run. You wouldn't do that so why do that to the most important sensor in your fuel and emmissions system.

First of all, In my humble opinion, I believe that if it ain't broke, don't fix it. I did a little bit of research on O2 sensors and concluded that there was no clear evidence about the life span of an O2 sensor but a lot of opinion. In my case, there were very few miles accumulated under RICH mixture conditions before I fixed the problem. If anything suffered, it was probably the Cat.converter. It's downstream from the fuel mixture equation but might have some feedback effects.

Preventive maintenance should also be considered from a cost effectiveness standpoint. I've replaced many O2 sensors (using the appropriate tools and technique) and know that it's very easy for something to go wrong and mess things up. Also, I've never noticed any difference in performance after replacing one. That's my experience!

I ask myself, why pay a lot of money (O2 sensors aren't cheap) and risk breaking the fool thing in the pipe just because of an unproven "preventative maintenance" habit which I don't share.

I'm not saying that I'm right and you're wrong. I just have a different opinion of how to spend the money, and risk of something going wrong (including burning my fingers or hand) versus the nebulous benefit of increased efficiency.

First of all, In my humble opinion, I believe that if it ain't broke, don't fix it. I did a little bit of research on O2 sensors and concluded that there was no clear evidence about the life span of an O2 sensor but a lot of opinion. In my case, there were very few miles accumulated under RICH mixture conditions before I fixed the problem. If anything suffered, it was probably the Cat.converter. It's downstream from the fuel mixture equation but might have some feedback effects.

Preventive maintenance should also be considered from a cost effectiveness standpoint. I've replaced many O2 sensors (using the appropriate tools and technique) and know that it's very easy for something to go wrong and mess things up. Also, I've never noticed any difference in performance after replacing one. That's my experience!

I ask myself, why pay a lot of money (O2 sensors aren't cheap) and risk breaking the fool thing in the pipe just because of an unproven "preventative maintenance" habit which I don't share.

I'm not saying that I'm right and you're wrong. I just have a different opinion of how to spend the money, and risk of something going wrong (including burning my fingers or hand) versus the nebulous benefit of increased efficiency.

Do what you want.

O2 sensors are about $40 and swapping them out takes about 10 minutes.

Replacing O2 sensors is an "unproven" maintenance technique?

Comon now, that is you opinion, FACT has it being one of things you DO replace to keep your engine running as efficiently as possible.

Even a 0.5MPG difference in fuel economy will have you paying for those O2 sensors in no time flat especially with fuel prices climbing by the day.

I replaced mine and had a good 1-2mpg increase in economy and my engine was running fine and smooth before the swap.

It just now runs fine and smooth with more money in my wallet at the end of it.

........Even a 0.5MPG difference in fuel economy will have you paying for those O2 sensors in no time flat especially with fuel prices climbing by the day.
........

The amount of money you save depends.........

If I assume a modest but probably realistic improvement of 0.5MPG, 17MPG Ave for city driving (that's what it says on the window sticker which I still have), $3.15 per gallon last time I bought gas, 20 Gal. tank capacity, $57.90 (tax included) for an O2 sensor (Bosch) at the local Napa store, and here's the big depends factor: an average of 100 miles per week of city driving.

If I did the calculations right, no time flat for this car is about 26 months to break even. What! me worry?

If you're doing 100 miles per day, it's a different story.

I'm surprised to see that O2 sensors have come down in price. The last time I replaced one on my Buick, it cost 95 bucks.

The amount of money you save depends.........

If I assume a modest but probably realistic improvement of 0.5MPG, 17MPG Ave for city driving (that's what it says on the window sticker which I still have), $3.15 per gallon last time I bought gas, 20 Gal. tank capacity, $57.90 (tax included) for an O2 sensor (Bosch) at the local Napa store, and here's the big depends factor: an average of 100 miles per week of city driving.

If I did the calculations right, no time flat for this car is about 26 months to break even. What! me worry?

If you're doing 100 miles per day, it's a different story.

I'm surprised to see that O2 sensors have come down in price. The last time I replaced one on my Buick, it cost 95 bucks.

Still you will be reaping the benefits during and after the 26 months seeing you won't be running slightly rich which will foul your plugs, wear your cat out faster and overall won't be getting the potential performance out of your truck.

$57 and you are saying that is too expensive? It will last over 10 years with the mileage you will be using it for.

For everyone else that drives more than that per week, they will be reaping the benefits so much sooner and keeping the money in their pocket where it belongs.

An ounce of pervention is equal to a pound of cure and being cheap will only cost you more in the long run.

Still you will be reaping the benefits during and after the 26 months .................in the long run.

At my age, 26 months could be the long run.:wink:

Well, I went with a Delco replacement O2 sensor. It was 70 bucks my cost, Wholesale since I have a Body Shop. What an amazing difference it made. Almost as much as replacing the CPI unit did way back when.

I tracked the EGR Duty Cycle and kept tabs on the O2 cross counts. The Rich flag was on more 80% of the time as well. My cross counts were way low. On a 7 mile trip home I might hit 20 or so. My Duty Cycle for the EGR would work during acceleration from idle for a short while, then quite.

After replacing the O2 sensor, my cross counts went off the charts. The mVolts for the O2 sensor were ranging from 50 to 996, never before had I seen such activity from the O2 sensor in this vehicle. The EGR Duty Cycle is constantly active between idle and full throttle now.

Upon initial startup I could tell the engine was smoother and quieter. After a short 4 mile test drive, it was running silky smooth at idle, like never before.

The O2 sensor I pulled was a Bosch, wouldn't have any idea how long it had been in there.

Well, I went with a Delco replacement O2 sensor. It was 70 bucks my cost, Wholesale since I have a Body Shop. What an amazing difference it made. Almost as much as replacing the CPI unit did way back when.

I tracked the EGR Duty Cycle and kept tabs on the O2 cross counts. The Rich flag was on more 80% of the time as well. My cross counts were way low. On a 7 mile trip home I might hit 20 or so. My Duty Cycle for the EGR would work during acceleration from idle for a short while, then quite.

After replacing the O2 sensor, my cross counts went off the charts. The mVolts for the O2 sensor were ranging from 50 to 996, never before had I seen such activity from the O2 sensor in this vehicle. The EGR Duty Cycle is constantly active between idle and full throttle now.

Upon initial startup I could tell the engine was smoother and quieter. After a short 4 mile test drive, it was running silky smooth at idle, like never before.

The O2 sensor I pulled was a Bosch, wouldn't have any idea how long it had been in there.

Glad it worked out for you.

The O2 sensor is a tune up item that is often overlooked or people think it never has to be changed until the check engine light comes on which is wrong.

Glad it worked out for you.

The O2 sensor is a tune up item that is often overlooked or people think it never has to be changed until the check engine light comes on which is wrong.


Yes! I still have a hard time believing how badly this Blazer once idled and how much bog and hesitation it had during off idle acceleration without throwing a code.

I have a '93 and the last time I changed the 02 sensor, it was a unbelievable difference in the quality of idle and acceleration the truck had. It never showed a code either, but just ran poorly. The difference with the older trucks basiclly is that the emissions system is not as capable as the newer OBDII systems, so to help make the OBDI system as effecient as it possibly can be, the O2 sensor is an important piece that has to be changed at somewhat regular intervals.