I have read a lot of threads stating that a failed O2 sensor will cause poor MPG's. Does a failure normally set a code in the computer and turn on the Check Engine Light?

Just curious. My MPG is in the 14 range and I feel it should be better.

 

An all out failure will set a CEL, but O2 sensors gradually get lazy the older and more used they are. It is possible that your sensors have just gotten lazy and are causing the bad fuel mileage. The only way to properly diagnose a lazy sensor is to put a scan tool or an osciliscope (sp?) on that can monitor the signal voltages from the sensors themselves.

Also, depending on where you live, your fuel may have changed to a 'winter mix'. I have found that in NY, my winter mileage is no where near what it is in the summer and I do not warm up any of my vehicles. Literally nothing changes between summer and winter besides the outside temperature. My Bravada was getting 18-19mpg after I had fixed all of the problems with it, but now that winter has set in, I'm getting 15-16mpg, and I see the same drop on my Bonneville of about 3mpg difference from my last summer average and what I'm getting now.

 

Hmm, that's interesting..... do you mean that the gas they sell in winter is a different mix? I have noticed a drop in mileage as winter sets in, and I don't warm up the vehicle either, but I live in western Washington state where there is not such a seasonal difference in temperatures, so it seems less likely that they would do anything different to the gas. So I may have other things going on. My O2 sensor was replaced about 18,000- 20,000 miles ago, I think, along with the EGR valve.

 

I had posted awhile back on the things that affect fuel mileage... I'll have to see if I can dig it up. A large factor in fuel mileage is the type and weight of your tires. Part of the difference with my Bonneville is that my summer tires are quite a bit lighter than my winter ones. Another big factor is maintaining proper inflation in your tires. Inflation should not exceed the maximum pressure listed on the sidewall in any event, but should always be at or slightly above the recommended tire pressure listed on the door sticker.

What year blazer are we talking about here? Depending on the year, you could have only 1 O2 sensor or you could have 3... IIRC, after '96, all blazers had 3 O2 sensors (but it could be '97). If you have more than one sensor, then whats to say that the other sensor hasn't gotten crappy (they normally don't go bad one at a time anyhow).

I'll await your reply with the year before further speculating.

 

It could be as simple as your wire has fallen off the O2 sensor. Typically when a O2 sensor is failing it will cause the truck to run rough, and a bunch of other crap. A simple test to determine if the O2 sensor is failiing is to disconnect the wiring from it. The puter will go into a preset run mode and the engine will run great, however your mpg will be horrible.

When was the last time you gave the truck a tune up?
Improperly gap plugs also cause it to have horrible mpg, as does a fualty temp sensor, and a few other sensors.

Check the codes, not all of them will trip the CEL (check engine light).

 

Well, I'm not sure if people are replying to me or to the thread-starter, but my S10 Blazer is a '91 and I think (but am not positive) that it only has one O2 sensor. Correct me if I am wrong! But I think we still don't know the year of Tom's Blazer.

I did get the truck a full tune-up about 1500 miles ago, except that the fuel injection cleaning was not done, and I'm wanting to do that pretty soon. Could that affect the mileage, too?

I do use the more lightweight passenger tires and responded yesterday to a post in the Tires forum about that. Will be going there next. I'm sure that inflation could be an issue, as well as the fact that I carry a lot of stuff in the truck most of the year.

My MPG is around 16 right now, not as bad as Tom's 14, and I'll see if unloading the truck and filling up the tires makes a difference.