Check engine light codes for a 2005 pontiac Bonneville 3.8 V6

Every year at this time I get check engine code about either the temp. sensor or the thermostat. I replace the temp. sensor every year, and last year I replaced both the temp. sensor and the thermostat and flushed the cooling system and replaced the coolant. but not this year, no I’m not doing it! For there has to be some other reason other than the temp sensor failing each year!
is any one else experiencing this?
Has any one discovered the actual culprit to this?

Looking to hear from you.


A low coolant level will also cause that code. Specifically, if the coolant level is below the level of the sensor.

Here are the parameters for setting codes P0125 and P0128. Please read page 8 and 9.
Hopefully, this’ll help you diagnose the cause of this recurring problem.

If the coolant level is low when this DTC sets, you may want to consider the possibility of a leaking intake manifold and/or gasket. Apparently, that was fairly common on the 3.8 V6

@flywelder sorry, I forgot to post the link. Here it is.

which temp sensor have you been changing. this car may have two. one for the computer, one for the guage or light. also, be sure to open the air bleeders when you fill the coolant, so you don’t have a large air pocket inside the engine. how’s your heater working?

I had the same problem with my 2001 Bonneville. I just kept clearing the codes and when the weather warmed up the problem went away.

All great ideas, thank s to all. I checked with the auto shop and there is just one temp sensor for my 3.8 L engine. and located below the thermostat housing. I bled off a quart of coolant to release any trapped air.
Coolant level is between the low and high marks on the recovery tank.
I looked for coolant leaks at the manifold and saw None.
My heater works very well, and provides heat within mins. of starting the engine.
In case the codes were old, I then cleared the memory of all codes, and no engine codes were set for 250 miles then just tonight the check engine light came back on, and tomorrow night I will have the codes read and find out what they are.

Does any one know what type of fluid to use in the tranny of my 2005 Bonneville with 3.8 L ?

@flywelder look on page 6-25

ordinary old Dexron 3