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p0128 coolant temp below thermostat regulating temp ?

12-12-2007, 08:30 PM
I have 2001 chevy venture 3.4L OBD2 code p0128 and p0128 pending code. it seams to pointing to the thermostat or engine coolant sensor. I don't think the thermostat is stuck open becuase the heater is working normal. But the temp gauge is staying below the lowest dash mark however it will go between the frist dash (0) and about 1/4 mark for a if i erase the code or remove the negitive battery lead. But it will start going below the lowest dash mark again after driving it for 12 miles or so on the freeway and about 20 miles later the cheak engine light comes back on. I have 2 questions on this issue? 1): Does this sound more like the sensor or the thermostat or both? 2): Will i hurt my van if i drive it with this condition. i was want to do some traveling over the holidays 700 miles or so. Any input would be a great help.

12-13-2007, 11:27 AM
Can't tell for sure which one it is yet, thermostat or sending unit. One way to check would be to measure the coolant temp at the radiator filler opening and compare to the dash gauge. If the temp follows the engine thermostat, it's the sending unit. If it follows the sending unit, it's the thermostat.

If it is a thermostat issue, driving this way might cause some problems. The engine will never get up to the proper temp and the fuel system will try to overfuel, possibly causing problems. At least your mileage will suffer.

12-18-2007, 08:41 PM
I would vote for the thermostat, because I have the same problem right now. But, to be sure, I would use a thermometer (infrared type) and measure the temperature right at the Engine Coolant Temperature (ECT) sensor, which is mounted right near the thermostat housing.

I did the same thing on my van, and the temperatures matched up very well. By the way, if you want to see what the temperature gauge would look like with number on it, click the link below.

Temperature Gauge with Numbers (

Take Care,

12-18-2007, 10:20 PM
wellcome to AF: here is the link to a heating problems Heater problems. Help!! (

16th hippy
12-20-2007, 06:47 AM
if there was a problem with sensor then would be a code for sensor. 99.9% of the time with a p0128 the thermostat is at fault. the thermostat can be stuck open a bit and heater still works, but pcm knows that engine is not warming up all the way because of what ect is telling it. replace thermostat.

01-12-2008, 01:36 AM
and do yourself a favor put one made in usa in . might have to look all over but worth it.

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