P0128 Engine Running too cold?

[Dusty_GMT800]

Got a CEL with the code P0128 saying that my 02 Tahoe is running too cold, I think it may be the radiator cooling fan is running too often, maybe stuck on or something. Does anyone know where the fuse is located? If it isn't this then my other guess is a stuck thermostat.

 

[Rocket Man]

If the fans are electric, you can unplug them at the bottom of the radiator. Each fan has a plug. It could also be the coolant sensor or the thermostat.

 

[Dusty_GMT800]

If the fans are electric, you can unplug them at the bottom of the radiator. Each fan has a plug. It could also be the coolant sensor or the thermostat.

I don't think they're electric, did the 02s come with electric fans? Thought that was a thing on the newer models. My next step is the thermostat, I think it could be stuck open not allowing the engine to heat up enough.

 

[Doubeleive]

02 didn't have electric fans, I think they started using them on some 04's or 05', I would check the coolant sensor first before digging into a bigger job, you could probably just run your heater to crank it all the way up and see if it feels normal hot or just stays luke warm that would probably be the easiest way to go about it, you should be able to tell that way.

 

[Rocket Man]

I don't think they're electric, did the 02s come with electric fans? Thought that was a thing on the newer models. My next step is the thermostat, I think it could be stuck open not allowing the engine to heat up enough.

02's have clutch fans ; I was wondering why you were looking for the fuse unless you did an electric fan conversion like I did. Clutch fans don't have fuses or anything you can unplug but you can check to see if it doesn't turn on until the engine is up to temp. If it starts running before that it's a problem. Then you'd be left with thermostat or sensor.

 

[Doubeleive]

if you didn't add electric fans and it has a stuck fan clutch that could another reason, the fan should spin with some light/med resistance, if it feels pretty tight or doesn't move then its probably bad.

 

[Doubeleive]

02's have clutch fans ; I was wondering why you were looking for the fuse unless you did an electric fan conversion like I did. Clutch fans don't have fuses or anything you can unplug but you can check to see if it doesn't turn on until the engine is up to temp. If it starts running before that it's a problem. Then you'd be left with thermostat or sensor.

beat me to it, lol

 

[Dusty_GMT800]

02 didn't have electric fans, I think they started using them on some 04's or 05', I would check the coolant sensor first before digging into a bigger job, you could probably just run your heater to crank it all the way up and see if it feels normal hot or just stays luke warm that would probably be the easiest way to go about it, you should be able to tell that way.

I run the heat and it's definitely colder than it used to be, I ordered an AC Delco Thermostat on amazon, should be here Wednesday. Also have a couple gallons of Dex-Cool So I'll change out the coolant. I think it may be that the thermostat is stuck open since the truck gets cool when I am on the highway.

 

[Dusty_GMT800]

If the coolant sensor has gone bad, would it not read at all? Or just read incorrectly? If it's a cheap and quick thing to do I'd rather do that before pulling the thermostat and flushing my radiator. Looks like the sensor is about 15 dollars on amazon. This morning my heat seemed warm but not sure if that was just in my head since it was 40 degrees out.

 

[Rocket Man]

You don't lose much coolant changing a thermostat, and it's super easy. Maybe a gallon of coolant, and don't forget new gaskets. You could probably re-use the old ones but why take a chance. Either way.

 

[Doubeleive]

If the coolant sensor has gone bad, would it not read at all? Or just read incorrectly? If it's a cheap and quick thing to do I'd rather do that before pulling the thermostat and flushing my radiator. Looks like the sensor is about 15 dollars on amazon. This morning my heat seemed warm but not sure if that was just in my head since it was 40 degrees out.

the sensor would not make it run cold it would only give a false reading if it was faulty, running cold is either the thermostat is stuck open or the fan is doing major overtime.

 

[Dusty_GMT800]

I meant that if the sensor was faulty would it cause my Engine Temp Gauge to read too low or just never go up at all? As of now it sits below 185 when reading the gauge in my dash, but during the last week when the weather was warmer I didn't really have an issue. Only seems to happen on cold days which is why I'm leaning towards it being the thermostat stuck open.

 

[Doubeleive]

your gauge should always read the same temp unless its really hot out and your idling or your towing or driving it pretty hard it may go up a little then, but it shouldn't be cooler unless the engine was just started and it hasn't got up to normal temp yet, but again the sensor would only give a bad reading it wouldn't actually make the motor run cold it's just a sensor its doesn't do anything other than give you information.

 

[Dusty_GMT800]

Gotcha, on my way to work there was a decent amount of traffic before the highway, the truck warmed up to about 185 before I got on the highway according to the gauge. However, in the morning I got onto the highway without much traffic and the truck never reached the normal operating temperature. Just curious, the normal operating temp is 185 correct?

your gauge should always read the same temp unless its really hot out and your idling or your towing or driving it pretty hard it may go up a little then, but it shouldn't be cooler unless the engine was just started and it hasn't got up to normal temp yet, but again the sensor would only give a bad reading it wouldn't actually make the motor run cold it's just a sensor its doesn't do anything other than give you information.

But if the sensor is faulty it could cause the engine to believe it is running too cold is that what you are saying? Does seem like the engine is actually running too cold sometimes since when the coolant temp on the dash is below 185, the heat is not as hot, however, when it is up to normal operating temperature the heat blows hot.

 

[Doubeleive]

Gotcha, on my way to work there was a decent amount of traffic before the highway, the truck warmed up to about 185 before I got on the highway according to the gauge. However, in the morning I got onto the highway without much traffic and the truck never reached the normal operating temperature. Just curious, the normal operating temp is 185 correct?



But if the sensor is faulty it could cause the engine to believe it is running too cold is that what you are saying? Does seem like the engine is actually running too cold sometimes since when the coolant temp on the dash is below 185, the heat is not as hot, however, when it is up to normal operating temperature the heat blows hot.

Well that's why I suggested feeling the heater, unless you want to try and trouble shoot the sensor that's the second best way to tell, if the temp says low and the heater is not as hot then it's likely the thermostat

 

[Dusty_GMT800]

Well that's why I suggested feeling the heater, unless you want to try and trouble shoot the sensor that's the second best way to tell, if the temp says low and the heater is not as hot then it's likely the thermostat

Heater definitely feels warm when the truck is actually warmed up, my new thermostat and housing comes in Wednesday (thank you amazon prime) How involved is the coolant sensor change? I may as well just change it out for a new one if I am already going to drain and flush the radiator anyway.

 

[Doubeleive]

should be in location #6 or in the same vicinity

 

[Doubeleive]

eh that's a pretty crappy photo, its here

 

[M1Gunner]

You would know if the ect sensor was bad. The signal to the pcm would read -40 and the truck would be extremely hard to start if at all considering how rich the fuel trim would go.

Engine running too cold... stuck open thermostat.

 

[Dusty_GMT800]

should be in location #6 or in the same vicinity

View attachment 186624

Got it, so looking at the other picture you sent it's on the passenger side under the plugs. Shouldn't be too hard to replace if I have to do it.

You would know if the ect sensor was bad. The signal to the pcm would read -40 and the truck would be extremely hard to start if at all considering how rich the fuel trim would go.

Engine running too cold... stuck open thermostat.

Well it starts up just fine, so I guess it's the thermostat, new one comes in today, hopefully I have a chance to do it sometime this weekend.