Hello All,

My sons 2001 Malibu has been setting code p0128(open thermostat not allowing engine to heat properly). Weird thing about this one is, when code sets, heat gage completely stops working. When I erase the code the gage works again untill code sets again.

Does this mean that the heat sensor also quits working? The reason I ask is because when this happens his rpms go wild and gas mileage goes south. So why not just change the thermostat (I'm sure you're asking) Well, GM thought it would be a good idea to run the crossover manifold directly over the thermostat leaving no clearance to remove thermostat without removing manifold pipe. I know sooner or later he is going to have to get this done. He can get away with it during heat of summer because engine will heat up quickly, but it still needs to be fixed.

My main concern here is why would gage and sensor quit working when it sets code and is something more going on here?

Thanks for any input.

Well, more often than not, the code sets because the thermostat is opening too soon and it takes too long for vehicle to get up to normal temp..........

The other possibility, is the coolant temp sensor, circuit, itself.....it could be the sensor, the PCM, or the wiring in between......

A bad thermostat opening too soon acts like this, if you let it idle, it will get up to temp....as soon as you drive down the highway, and that cool air flows over the radiator and engine, that temp will drop from 190-200F, to possibly 160-170F...........

And by the way, the easiest way to change the thermostat, is to relieve the pressure, and then remove the throttle body(you will need a new gasket)....with the throttle body out of the way, it'* easier to get at the two thermostat housing bolt and stud....you do not want to remove the crossover pipe.....I guarantee those exhaust studs will snap.....

I hadn't thought about removing the throttle body. That may do the trick. You're right, I don't want to remove pipe for the very reason you mentioned. By the way , the first thing I did was replace heat sensor. I just assumed the thermostat was stuck open when that didn't help. I'll take another look at that throttle body and go from there.

Thanks for that

No need to remove the x-over pipe. The smaller hot water tubes need to be moved out of the way though. It is pretty easy to get the thermostat housing in and out of there......If I remember correctly, the lower rear thermostat mounting hole is actually slotted, so you actually only nreed to loosen that back lower bolt, with the other bolts removed, the thermostat housing should slip upwards and away from the motor.

We have experienced this many times it just took a new temp sending unit to fix it. The RPM goes wild from wrong reading its sending the engine ECM
The temp sending unit tells the engine ECM the temp of the engine
and the gauge.
Because the RPM goes wild it shows there is no wiring trouble.
So to save your self lots of time and trouble replace the temp sending unit

Thanks again to all

I replaced the head gasket on a God Awful, troublesome Chevy Trailblazer.....after buttoning it all up, I started the engine and it idled terrible....fluctuating between 500 and 1,100 rpm'*.....I thought I blew it with the HG replacement,. Then I noticed I had no engine temp reading on the guage.....With the engine running, I plugged the wires back onto the temp sensor and the engine immediately began running perfect. I asked my brother (ASE certified engine driveability tech) why the engine would run so poorly with the sensor disconnected, he told me that when the ECM receives no input from the coolant temp sensor, the ECM runs the engine as if the coolant temp is at -40*F.

You would think GM would program a default temp of 75*F or something like that.