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PO128 Coolant Thermostat code fix


Oldsmobile Alero, 2004, 3.4L V6.

I noticed a few days ago that PO128 had popped up and looks like its been tripped a while. I have another code PO401 that I have been tackling for months to no avail, so check engine lights been on non-stop and I missed this new code popping up. I think its been up for a few weeks. Here is a link to the code:

I cleared it and it hasn’t come back yet in two days, but I’m keeping a look out.

The code appears to mean that the car thinks my coolant temperature is not getting hot enough after a certain period of time. Says it usually points to a thermostat issue.

Thing is, the car seems to be working fine. Now that I’ve been paying attention I feel like the temperature gauge is resting a little lower than it used to. Usually within 10 minutes or so I hover around the middle. Now it takes a little longer and I hover around 3/8. I think my heat was SLIGHTLY weaker, but I also may just be tricking myself.

So seems like the cooling system is working too well? Downside would be heat is worse potentially? Any other actual issues of the coolant staying a little colder?

Regardless, I want to fix it. Thing is I feel like thermostats are all or nothing. This doesn’t seem to be stuck open or closed in my mind, or I would overheat or be always cold right? So maybe its one of the other parts the article suggests: ECT or IAT sensor?

Another question. I have a hard time finding youtube videos for my car. I’ve found other videos with similar under the hood 3.4L I think Monte Carlo rings a bell. Anyone know a nice list of similar cars I can search for when trying to find repair videos for my olds alero 2004 3.4l?

My money is on the thermostat. I’ve had that exact issue - gauge at 3/8, just not quite there. When I removed my t’stat, I tested it in a pot on the stove with a stick thermometer. I had to cycle it open and closed several times, but I finally caught it just barely not closing/seating; allowing just enough leakage to set my check engine light.

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Ok cool.

Sounds like from what I have replacing the thermostat in general is a good idea simply because it does fail eventually and in the case of the opposite failure, being closed, your car overheats and is unusable.

So any advice on a good walkthrough or video for my car how to replace it? I’ve heard its easy on some, hard on others, and you should drain your coolant before hand (think I’ll do a coolant flush at same time, car could use it).

Like I said I constantly get nothing looking for 3.4l 2004 alero on youtube, must have not been very popular, but I find other cars sometimes that look nearly the same under the hood. I would love a full list of them I can search through when I have a problem that are similar.

Replace your thermostat with the correct temp model for this car. T-stats fail open causing the car to run cool. Yours is failing and this will only get worse. Running cool all the time is essentially like taking short trips that don’t fully warm the car. Moisture doesn’t get removed from the oil, fuel economy is poor, ect.

As for YouTube… the Alero shares its platform (GMX130) with the Chevy Malibu and Pontiac G6 so maybe those will have more submissions.

Hey thanks!

Is going to rockauto and selecting a premium grade for my make and model good enough, or is there something specific I should look for to get the proper temp model? I’m assuming what you mean is thermostats are built at various temperature ratings, and I need to get the proper temperature rating for my vehicle. (full disclosure I have never replaced a thermostat).

Yes, exactly. While there can be several different temperature values available for your car, pick the one the car came with. Rockauto shows 195 degrees as OE (original equipment) and has one for 180 degrees listed as an alternate. Choose OE or that code won’t go away. Been there, done that!

It isn’t hard to change a t-stat. Get a drain pan and drain the radiator a bit to get the coolant level under the t-stat location OR set the pan under the t-stat housing and let it splash out when you break the gasket seal. Messy, but you won’t risk breaking the drain petcock. NOTE WHICH WAY THE T-Stat is installed before you remove it! Take a picture, make notes, something to remind you because backwards installs won’t warm the car up. Replace the new one exactly the same way with a new gasket or o-ring.

Refill the coolant with what your owners manual says but note the color. Your car is old enough to have had its coolant changed to green -universal instead of Dex-Cool orange. Don’t mix colors. Add fluid and leave off the radiator or resevoir cap while the engine warms up with the heater on full hot so you can add fluid as needed. Once the level is set, put the cap back on and check it again in a day or 2, add as needed.

It’s best to use the original brand thermostat (OEM).
I’ve always replaced them every 6 (in the old days of 2 year coolant) or 10 years, along with the radiator cap when doing a coolant change.
It’s usually pretty easy to do, just two small bolts.
Thermostats occasionally fail closed. I had this happen once on the highway.
It was a BAD SCENE, but luckily that '76 Chevy Nova had a tough 6 cylinder.

Ok great thanks everyone!

Think I have all the info I need now.

I’ll come back and let you all know how it went once I get the part and replace it.

For the record, the P0401 code is an insufficient flow of the EGR system. You need to clean the orifice at the throttle body and the EGR valve. Hopefully that’ll stop the code.

Note that the EGR’s function is to prevent the cylinder temps from getting too high by allowing a bit of inert exhaust gas to be drawn in with the incoming air. It displaces a bit of the oxygen, cooling the combustion process a bit. The oxygen in exhaust gas is already bound up with carbon and nitrogen, so it cannot feed the fire. An analogy would be closing the vents some in a woodstove, reducing the oxygen availability to lower the flames.

I think Stant make excellent thermostats and radiator cap. I would not buy a thermostat from Rockauto, the shipping costs would make it more expensive than a local parts store. I wlll admit, I never change mine until they act up. On my own family’s cars I have replaced maybe 3 in 60+ years and I bought a lot of cheap cars that had a hiatus at my house before the junkyard.

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A T-stat can fail by simply opening too soon. This happened to the original one on my '90 Camry. In a pot of water on my stove, it would open at about 150 degF instead of 190 degF.

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Yeah, you couldn’t kill that Chevy 6 with a sledgehammer…