P0128 code for a LS nonbeliever ('15 5.3 Silverado)

[pagrunt]

Well I have to dive into this with my '15 Silverado but figured it might be something others may have pop up on their LS powered G. After driving home from work tonight I had both fans kick into full scream as I was parking the truck & noticed the temp gauge sitting only at 160 with no CEL on. Scanned & got a pending P0128, everything under the hood was cool to touch, overflow at the time was not at the full cool mark but since the truck was running for about 20 minutes at around 55 on the ride home I want to recheck in the morning as everything will be at a near air temp & cold. Inner web says culprit could be between the thermostat, temp sensor near it, low coolant (morning recheck) &/or fans themself. Tempted to try the stealership but the one local refused to accept my Ally extended warranty & not really in the mood to deal with fight with others to see if they'll accept it or not as we have an unexpected family thing just happened to add to getting this quickly done. Besides ensuring the coolant level is correct should I just swap the thermostat & sensor & go from there & just the thermostat? To be honest I'd rather throw the $100 at the garage & let them trouble shoot & let Ally cover everthing but speed of gettig it fixed is the primary course of action.

 

[Sweet_Johnny]

I found this Reddit post that says it's likely just the thermostat and not the sensor, considering your coolant level has been verified. The OP had a Dodge and the shop warned of overheating but apparently that's not possible with that code. All new to me.

 

[pagrunt]

I've been reading posts & watching videos to night & keep seeing it leans to this. The odd thing to me is I'm use to hear (& experiencing) low coolant causing over heating, not over cooling & I've come across several running with this. Another kicker is thermostat & coolant runs close to my deductible.

 

[Sweet_Johnny]

I don't claim to understand the new stuff but I think the only way low coolant would throw a low temp code is if it never, ever touched the sensor. I'm used to being low enough to get hot but still high enough to wash the probe in the cylinder head. I feel that you'd have to be damn near empty to get a code like that, pointing me to a bad sensor. But that doesn't jive with the Reddit tech's experience. Maybe change the 'stat and if it persists suspect the sensor?

At least it's too cold and not too hot, but I have no idea what changes the ECU makes based on that.

 

[-dw-]

Our shop have replaced a few thermostats for that code. It’s a easy one to replace.

 

[pagrunt]

I seen that it's an easy to do job but sucks I'm seeing the thermostat locally is looking to be $68. Now It's a cost & schedule thing on it being me or a shop.

 

[Streetbu]

Tstat sticking open is a very common issue on those trucks. Literally a 15 minute job. Get an AC Delco replacement. Had it happen to my 16 Silverado

 

[pagrunt]

Right now I haven't found an on line listing at a local parts store that list AC Delco replacement. O'Reilly is the only one not showing a store brand (Murrary) Napa is the cheapest by $10. Still temped for the temp sensor too as I'm be in that spot in that spot.

 

[69hurstolds]

Delco tstat all the way. On a stock engine don’t get stupid with a 160 degree deal or anything. Go with what GM recommends. That’s my 2 pennies. Anytime I change coolant I overkill and put a new one in on those LS engines because they are super-easy to change.

 

[doood]

I've had two P0128 thermostat repairs on two different FIAT/MOPAR in the last year. One I did myself, the other I let the shop do because was tucked out of the way.

I don't know if older designs from like Gen1 SBC era used to fail closed, leading to overheating, but the modern thermostats seem to fail open, safely keeping the engine colder and the ECU in open loop.