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2003 chevy trailblazer_lt_ext
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Discussion Starter · #1 ·
Quick background: Got a code while towing my trailer in mid-May. Mileage went in the toilet. From 13mpg to 6mpg. Cleared the code and all was fine for a month. On the way home from a trip yesterday, towing again, mileage went from 13 to 9 after filling up half way home. A little while later, the SES light went on again. The codes are P0171 & P0172. 107k miles. Runs fine, plenty of power, no hesitation or anything else strange. If not for the poor mileage (and sudden change too!) and SES light, I would not know that anything was wrong.

After some reading, I will be changing the fuel filter, clean the throttle body, and add fuel injector cleaner. I'm also going to look for vacuum leaks. Anything else I should be looking at? I did buy an 02 sensor. Should I take it back? Or put it on anyway?
 

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2003 chevy trailblazer_lt_ext
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Just a thought - but with that kind of mileage, you might want to replace both O2 sensors, especially if you are going to do everything else you listed. Also, when was the last time you changed the plugs? Might be a good time for that as well.

Just my :m2:
 

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2003 gmc envoy_slt_xl
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Quick background: Got a code while towing my trailer in mid-May. Mileage went in the toilet. From 13mpg to 6mpg. Cleared the code and all was fine for a month. On the way home from a trip yesterday, towing again, mileage went from 13 to 9 after filling up half way home. A little while later, the SES light went on again. The codes are P0171 & P0172. 107k miles. Runs fine, plenty of power, no hesitation or anything else strange. If not for the poor mileage (and sudden change too!) and SES light, I would not know that anything was wrong.

After some reading, I will be changing the fuel filter, clean the throttle body, and add fuel injector cleaner. I'm also going to look for vacuum leaks. Anything else I should be looking at? I did buy an 02 sensor. Should I take it back? Or put it on anyway?

Hi,

What's your truck milage?
If it's over 70,000 miles, change the cat.
 

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2005 chevy trailblazer_ls_ext
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Actually, I had my truck's cat defective and it ran fine with plenty of power. I recognised that it was defective only when it gave a code at steady low speed. I changed the cat and it's fine now.
Most of the time bad cats - or maybe a should say "plugged" cats show up as a loss of power. I've seen it happen NUMEROUS times over the last 35 years or so.
I'm not saying your condition can't happen.
 

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Discussion Starter · #8 ·
I'm also not sure it's relevant. Everything I could find on these codes suggests they are being thrown by O2 readings from the exhaust BEFORE it hits the cat. I've never had to change a cat due to age. In fact, I've never had to change one at all. And I drive most of my vehicles well above 150k.
 

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I'm also not sure it's relevant. Everything I could find on these codes suggests they are being thrown by O2 readings from the exhaust BEFORE it hits the cat. I've never had to change a cat due to age. In fact, I've never had to change one at all. And I drive most of my vehicles well above 150k.
Hi,
The cat is a chemical substance that starts a chemical reaction fast enough to change CO to CO2. It loses this feature as it gets old. Gases go through the cat and leave with a slight change or non. The O2 sesors ignite the SES light when they get the same reading. It is not necessary for the cat to be clogged to be defective.
 

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Discussion Starter · #10 ·
Well the light went off on its own yesterday. It will come back the next time I tow, I'm sure of it. The more I read up on this, the combination of the two codes is making me lean toward a faulty 02S. I'm still going to go ever everything and change the fuel filter anyway. It certainly won't hurt. I won't be towing again until mid-July, and that's a short trip, so we'll see what happens. We've got a 700+ mile trip in Sept, so I hope I can get this resolved by then.
 

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Discussion Starter · #11 ·
Update

Here's an update. I finally had time today to go ever everything. I found a tube on the air box that wasn't connected. A visual inspection never revealed it because the the hose sat flush. I didn't see it until I pulled the air box off. Evidently I didn't connect it when I changed plugs a couple of months ago. Thanks for all your help!
 
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