Hello all,

I am trying to figure out why my MPGs have benn going downhill the last couple months, the air filter could be part of it. But i took the ol' girl to autozone the other day because of a check engine light, they told me the code was P0128 which is the coolant temperature sensor. Could this be affecting my mileage as well? Also going along with that sensor could this be making my fan run all the time as well?

Thanks!

 

P0128 typically points to a bad thermostat being stuck OPEN. This does not allow the engine to warm up quickly enough, and when the code gets triggered along with the ECL, one will find the DIGITAL coolant temperature reading shows only "----", and the A/C will be disabled.

P0128 is one of the more common codes - use the search tool to look for p0128, and you will see half a zillion posts about it.

When the thermostat is stuck open, the ECM is probably setting the mixture high thinking the engine is cold still (which it is) - which will affect your mileage.

To verify it is your thermostat, set the DIC to display the Digital Coolant temperature, and monitor it as the engine warms up (ie: after it has sat overnight). You will likely see the temp climb to ~85F then blank out with those hyphens.

If you replace the thermostat, save some possible future hassles and just use a genuine GM AC DELCO one - and read this how-to on replacing the thermostat:
https://www.chevyhhr.net/forums/how-tutorial-library-21/how-save-$350-00-thermostat-replacement-2-4l-p0128-24493/

ps: the p0128 may come and go periodically as if you shut it off, and restart before it cools all the way down, it is already pretty much up to temperature - and it does not detect that it is slow to warm up.

 

Yes, that will cause bad MPG. (p0128) Like driving a '54 chevy with the choke on all of the time.
Have your spark plugs been replaced, 100,000 miles recommended. Withe A/C Delco Iridium.

 

Here is that link

https://www.chevyhhr.net/forums/how-...l-p0128-24493/

Please use the correct thermostat as mentioned and good old Dexcool!!
Park the HHR front end high when you refill the coolant to allow air to escape!

 

Yes the spark plugs were replaced around 115,000

 

So youre saying just replace the whole thermostat and be done with it?

 

Yes.

 

What do you mean by "whole". We are not meaning the housing and temp sensor. Just the t-stat.

 

Thermostat and change out the coolant if it's never been changed. 7 year old car should have had the coolant changed, but if not, or you're not sure it's been done, now is the time.
AND... Dexcool only !

 

Ditto to all of the above, especially the coolant change.

It doesn't matter if we're talking Dexcool or the "Olde Fashioned Green Stuffe", coolant breaks down into chemically corrosive compounds over time.

Hence the five year change interval, so change the thermostat and service the cooling system.

Then you'll have a happy HHR for years to come.