I'm running into the same problem, keep getting an SES P0128 code, fluid is fine, car only runs at
about 180-181 degrees.  Once the code is cleared, temp goes up (weather is rather cold now, hard
telling if this is a factor) to almost 210 when idle, but when driving, drops back to 180-181.  Am
leaning to prior mentioned solution, thermostat is stuck in open position, and willing to pay the 8.99
for the 192 OE Thermostat and do a flush of the coolant system to see if this resolves the problem.
Comments
I have the same problem with my car no heat and runs at 40 degrees could it be plugged heater
core?
If you mean the engine temperature is running at 40
degree, then the thermostat is stuck open ans needs to
be replaced.
It is your coolant temp switch and I bet your fans run all the time also, it is located below
thermostat housing.
any suggestions? I have a 2001 grand prix.. I also have a check engine light with this 0128
code. I have had the thermostat replaced as well as some sort of temperature sensor. Still
the light is on. I cant get my car to pass the inspection even to its running great.
There may be a service bulletin or computer update for your problem. Check with the
Dealer.
My 2003 runs for about ten minutes then stalls out what can be the problem??
Could be anything from a vacuum leak, an ignition
problem, or most likely a bad crank sensor or ignition
module. Any of these would need some diagnosing by a
qualified technician. Best of luck...
3-21-12
My 02 Grand Prix runs hot had new radiator thermostat and water pump car still running hot and
heater blows cold often and occasionally it will blow hot air. What's the problem?
Could be an internal engine problem such as a cracked
cylinder head gasket. Usually it's a bad sign when you
have no heat, or it only blows hot occasionally. A shop
can do a cylinder leak down test to find out what actually
is the problem.
4-16-12
Help! My 2000
Pontiac Grand Prix GT chocked and stalled out, would not re-start.  I got P0128
code. Will this code prevent engine from starting or there is other problems??
No, this code will not prevent your car from starting.
My 2003 pontiac grand prix gt temp gauge goes past half while sitting but goes down as i drive it.
There is no smoke or lights on. What could be the problem?
Check for a restricted radiator or low coolant level.
I had the same code on my 2005 and it was the ECT (engine coolant temperature) sensor.
Hello, I have a 2008 Envoy 4200 Vortec. The Temp gauge in side reads 100 F° all the time. The
A/C Compressor does not come on, But, the fan runs. Is this a bad coolant sensor ?? Can I check it
with a ohm meter, if so what resistance should I see at room temp. I am in Arizona so room temp will
be a little higher... Thanks for your help.
Yes, it does sound like the temperature sensor. Just tap
on it with a screw driver and watch to see if the gauge
jumps around.
My 2003 pontiac grand prix gt doesn't goes into reverse and I can't get a reading to find the code,
what needs to be replace?
If you dont have reverse, it is not something electrical.
There is an internal problem in the transmission.
I have a 2008 grand prix. The code I also got was PO 128 but my temp gauge is reading normal
and I have hot air. How do I get this light and code to reset?
Change the thermostat.
How much would this cost to replace the thermostat on a 2007 Pontiac Grand Prix?
Cost is approximately 1 hour labor.
Problem with my 2003 Pontiac Grand Prix check Engine Soon P0128.
Recently had engine cooling system flushed and coolant replaced.

Question :Just as my car turned over on 170,000 the
check engine light came on. I had
AutoZone check code. Came up p0128. Light stayed on for about 50 miles then went off. Any
ideas as to what this might be...or should I just leave it alone until it comes on again? Why does
this come on. Thanks a bunch.

Answer:
This code is described as 'coolant temperature below thermostat regulating temp or threshold'.

A common cause for this code is a sticking thermostat. The coolant temp is too low for longer
than the pcm(computer) expects. It is possible that there was an air pocket in the system after
the flush and that caused the code. Keep an eye out for the light to come back on, coolant temp
gauge (if you have one) to stay low, poor heat from heater/ac system, or coolant fans under the
hood stay on with car off. If some of these things occur, then have your thermostat changed, and
make sure the system is bled of air after re-filling the coolant.


2000 Pontiac Grand Prix 3.8 V6. Heater core flushed, hoses all checked all warm-no blockage,
thermostat changed-radiator checked-cap replaced- Question = I have a 2000 pontiac grand prix
GT-- I all of a sudden have nothing but lukewarm heat at best blowing. Also my car always runs at
210 but now doesn't get even close to that. We replaced the thermostat, had the radiator
checked and cap replaced. The heater core has been flushed and hoses checked all fine. My
husband said it heats up when sitting, but as soon as I drive it, well the temp goes down. My
husband has checked everything he could think of with the heating system. Can anyone offer
any suggestions? I'm one step away from taking it to the dealer. Which I'm sure will cost me a
bundle! Please help!

Answer:
You need to make sure there is not an air pocket in the cooling system. That will cause the
temperature to be erratic. Also, are you sure you installed the correct thermostat? If it is incorrect
or staying open all the time the car will not heat up to operating temperature and you won't get
much heat from the vents. You probably do not have a radiator, heater core, or hose problem
since the coolant temp is not getting to 210 like it used to. It sounds like a thermostat issue, so I'd
double check that to start with.

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Pontiac Grand Prix Code P0128- Coolant Temperature