Coolant not hot enough?

On way home in the Malibu and after getting through a road construction area with rough bumpy roads notice the check engine light is on.

Stop at parts store to get scanned and it comes back as the coolant isn't getting up to operating temp. In dash thermostat showing just below middle as it always is. Heat may have been a little hotter than usual, or it could have been my imagination.

Did flush and fill 18 months ago. As of now coolant levels are fine, no leaks noticed. Didn't notice any weird sounds and when I popped the hood didn't smell anything unusual. Figures I do 250 laps of VIR and it's fine but my drive home from work mucks it up.

I'm just starting to self learn automotive mechanics so I'm a bit clueless with this sort of problem.

Any ideas on what it could be?

Thanks in advance everyone.

A CEL for "coolant temp too low" is a new one on me!

I think the two most likely culprits would be

• coolant temp sensor reading too low (change sensor)

• Thermostat stuck open.

But I'm thinking the sensor is the more likely culprit.

Good luck!

my roommate's 04 Grand Prix throws the same code from time to time, and when it does the temp gauge reads low and the heater doesn't heat as well.. i told her that come spring time it's getting a throrough flush, new thermostat, and a refill with the proper green antifreeze in place of the Dexcool crap that's coating the whole inside of her engine and radiator with orange slime..

That is normally a bad thermostat. It could also be scale on the sensor that reads temp for the ECU or bad sensor. If the thermostat is easy to get to I would start there.

My Google-Fu found some info.

http://www.obd-codes.com/p0128

Possible Solutions

Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

•Verify coolant strength & level

•Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.

•Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.

•Verify proper intake air temperature (IAT) sensor operation, replace if necessary.

•If the above items check out good, replace the thermostat

•If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Other ECT sensor and circuit related DTCs: P0115, P0116, P0117, P0118, P0119, P0125

Novaderrik, yours sounds like the thermostat stuck open.

I think mine is a sensor since my heat and in car temp gauge are fine.

Hope it's the air intake sensor.

Is it slow to warm up? If the thermostat is lazy, but functions well enough to look ok on the gauge, the computer will get mad for emissions reasons.

My malibu dd did the same thing. Changed the sensor because it was cheap ($11) and easy to get to. Didn't fix it. Put a new thermostat and gasket in and flushed the red gunk to the traditional green stuff and no more light.

Sounds like when my thermostat went bad as well. My Malibu did the same thing, and my wife drove it all winter that way. Running rich for a few months killed the cheap aftermarket cat I had put on.

Bad thermostat. I've never seen a '128 thrown that wasn't a bad thermostat. Simple to diagnose, too... get car to operating temp at idle, drive around the block. If temps drop below what should be the thermostat's opening temp, the thermostat is bad.

Most GM cars have two temp sensors, one for the gauge and one for the computer. It could be the one for the computer is bad, thus the gauge one is still reading correctly. I would still consider changing the thermostat anyways, just to be sure.

Put a thermostat in it. The gauge isn't telling you truth, anyway- they are designed to make you calm- the needle will sit in exactly the same place on most modern cars from around 150 to about 225.

81cpcamaro wrote: Most GM cars have two temp sensors, one for the gauge and one for the computer.

Not true for a couple of decades.

Streetwiseguy wrote: Put a thermostat in it. The gauge isn't telling you truth, anyway- they are designed to make you calm- the needle will sit in exactly the same place on most modern cars from around 150 to about 225.

yeah, they're really nothing more than an idiot light … with a needle as opposed to a light

Streetwiseguy wrote:

81cpcamaro wrote: Most GM cars have two temp sensors, one for the gauge and one for the computer.

Not true for a couple of decades.

i think they got rid of the separate temp senders when they went OBD2 in 96- the gauge cluster is run by the ecm now, so whatever the gauges show is what the ecm is seeing.

81cpcamaro wrote: Most GM cars have two temp sensors, one for the gauge and one for the computer. It could be the one for the computer is bad, thus the gauge one is still reading correctly. I would still consider changing the thermostat anyways, just to be sure.

No, they don't, at least nothing new enough to have OBD-II will. They have one sender for the computer and then the computer gives the coolant temp information to the instrument cluster by CAN. Same for oil pressure and, IIRC, fuel level. (The computer needs to know fuel level so that it can run evap monitors correctly, and it is also factored in to the ignition timing strategy - it re-checks the knock properties of the fuel every time the fuel level goes up by a certain amount)

GM coolant temp sensors don't soft fail. They go from working to reading -40. A P0128 is practically always a bad thermostat, and I only say "practically" because there will always be that one weird case where it is not.

In short - it's the thermostat, you can count on that about as much as you can count on gravity.

Thanks everyone, have a day off coming up and I will take care of that and post how it went.

The only weird thing about your situation is that normally, a '128 will cause the coolant temp gauge to stay buried at full cold, no matter what the engine temp actually is. It gives a little more of an "oh crap" factor than just a check engine light, which people tend to ignore until the day before their vehicle's registration expires.

I guess they didn't put that little feature in your Malibu's code.

I once saw a P0128 tripped on a car with a 6 month old quality thermostat and full of fresh properly mixed coolant, though this was after it went on a trip in -25* weather. But if it lit up in positive temperature weather, I'd go with bad thermostat.

Eh, just put a piece of cardboard in front of the radiator.

Knurled wrote:

81cpcamaro wrote: Most GM cars have two temp sensors, one for the gauge and one for the computer. It could be the one for the computer is bad, thus the gauge one is still reading correctly. I would still consider changing the thermostat anyways, just to be sure.

No, they don't, at least nothing new enough to have OBD-II will. They have one sender for the computer and then the computer gives the coolant temp information to the instrument cluster by CAN. Same for oil pressure and, IIRC, fuel level. (The computer needs to know fuel level so that it can run evap monitors correctly, and it is also factored in to the ignition timing strategy - it re-checks the knock properties of the fuel every time the fuel level goes up by a certain amount) GM coolant temp sensors don't soft fail. They go from working to reading -40. A P0128 is practically always a bad thermostat, and I only say "practically" because there will always be that one weird case where it is not. In short - it's the thermostat, you can count on that about as much as you can count on gravity.

Forgot about the change on OBD2 stuff, been messing with OBD1 stuff quite a bit lately.

Thanks for the help everyone. Changed coolant sensor and thermostat and disconnected battery to reset cel And it's been three days without a code popping back up. Seal for the thermostat went bad and was allowing coolant to trickle past slowing down heat up time but still allowing car to eventually get to normal operating temp. That's why the heat still worked.