Thermostat P0128

04Trailblazer

Original poster
Member
Feb 27, 2012
53
i have a P0128 , ive been getting horrible gas mileage. I recently found out i had a P0128, i assume i have had a bad thermostat for about a year because the CEL been on for a year after i changed the non sealing gas cap... i plan on changing it this Mid march when i have the money. im just afraid of clogging up the Cat. how much longer can i run on a bad thermostat?
Thanks
 

Voymom

Member
Feb 3, 2012
2,523
04Trailblazer said:
i have a P0128 , ive been getting horrible gas mileage. I recently found out i had a P0128, i assume i have had a bad thermostat for about a year because the CEL been on for a year after i changed the non sealing gas cap... i plan on changing it this Mid march when i have the money. im just afraid of clogging up the Cat. how much longer can i run on a bad thermostat?
Thanks

I wouldn't try to drive it for to long. However I bought my Envoy a month ago and it had a bad t-stat in it when I test drove it on the lot. It only cost me $32.00 for a brand new OEM thermostat and it wasn't to hard to change. My truck didn't throw any codes though and my CEL never came on? Not sure if that matters or not. I don't know how long my truck was driven with a bad t-stat before we bought it, I had a back flow test done and it passed for not having any cat issues, but I do have an exhaust stutter....so I think I'm going to have it retested. If your t-stat has been bad for a year already, I would ASSUME you could very well have a clogged cat, that just seems like an awful long time to drive with a bad t-stat and not have any cat issues. But I could be wrong.
 

Blckshdw

Moderator
Nov 20, 2011
10,682
Tampa Bay Area, FL
04Trailblazer said:
i have a P0128 , ive been getting horrible gas mileage. I recently found out i had a P0128, i assume i have had a bad thermostat for about a year because the CEL been on for a year after i changed the non sealing gas cap... i plan on changing it this Mid march when i have the money. im just afraid of clogging up the Cat. how much longer can i run on a bad thermostat?
Thanks

Why exactly did you wait so long? :confused:
 

MedicatedMike

Member
Feb 24, 2012
101
Man the wheel bearing and thermostat fairies must be working overtime on premature failures....they on some new pixie dust reward system? :undecided:

We bought our TB less than month ago and changed wheel bearing last week and have come to realize that the therm was on the way out when we bought it. Gas milage was already suffering but didnt throw the code until a week after we bought it. Apparently the start to fail and open too soon, and over time open sooner and sooner, causing the engine to run colder and colder. At a certain point it usually throws the code when the engine runs so cold "enter majic number" for several trips in a row.

Anyway...point is...our TB is down to less than 12 mpg and thats not all city driving and we granny the chit out of it too! I expect it to go up dramitcally after swapping the water pump and therm as soon as it gets warmer. (No garage in Mi makes it tuff). Im also going to clean the throttle body, run a can of seafoam through, and then after everything install the new PCM tuned by PCMFORLESS. :biggrin:

**I realize I am taking risk of not replacing temp sensor at same time but I want to test my diagnosis. Also realize 02 sensor is likely in near future too....weeeee
 

04Trailblazer

Original poster
Member
Feb 27, 2012
53
Blckshdw said:
Why exactly did you wait so long? :confused:

well the very first time the CEL came on was for a bad gas cap. it then came on a week later again and i just assumed o its just the gas cap still , so 9 months or so i got the CEL checked again and it was a P0128 , i just got it checked a week ago and thats when i knew it was a P0128. so ive could of had a bad thermostat for shorter then a year. i do Plan on changing as soon as i get enough time and money to do it. I plan on changing it this March. Thank you everyone els! :smile:
 

MAY03LT

Member
Nov 18, 2011
3,420
Delmarva
The mileage in my 03 went from 21.2 to 15.2 (mostly highway, results not typical lol) when it had the p0128. It sounds like you already knew you had to do it asap, any claims of how long you could go before damaging the converter would just be a guess.

Mine ran me around $33.00
- gm tstat $15.00 ebay
- 1 gallon dexcool $16.00
- 1 gallon distilled water $.99
- 1 clamp to replace OEM lower hose clamp $.89
 

Mark20

Member
Dec 6, 2011
1,630
From reading up on th P0128 mine threw it can take a while. Specifically the code is for the engine not coming up to temperature within the time allocated. The computer also takes outside air temperature and distance driven into account. Mine went off on the a 20 mile or so trip home from NJ.
 

blazinlow89

Member
Jan 25, 2012
2,088
Before I did my oil change I had a CEL for the cam sensor (if I remember right), so I changed the oil it disappeared. Well the damn thing turned on again and now it is for the same thing. Time to order some stuff.
 

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