Low Coolant Temp

sc4808 · February 18, 2008

I Have a 2003 Sierra with the 4.3L V6, it's got 140K on it and has been one of the best trucks I've ever owned. Last week a problem popped up that I have yet to figure out. I got a check engine light and when I checked the code it was "low coolant temprature", now the first thing that popped in my head was the thermostat so I promptly replaced it, no change, after resetting the check engine light it came back on again after about 20 miles.

Then I was thinking that it must be the temprature sending unit, so I went to the parts store and found out that there are seperate sending units for the computer and the guage and since my guage is reading low as well what are the odds that both sending units are bad, slim to none I would think, so it must be something else, but what, I have been racking my brain and have not come up with anything.

By the way my guage is reading around 170 to 180 degrees, its hard to tell with the factory guage, but it is definately lower than it has ever been before.

And by the way, I live in the south and the temp for the last week has been in the 60's and 70's.

Any Ideas?

jrrod2004 · February 18, 2008

This may or may not help:

DTC P0711

Circuit Description

The automatic transmission fluid temperature (TFT) sensor is part of the automatic transmission fluid pressure (TFP) manual valve position switch. The TFT sensor is a resistor, or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, the resistance decreases, and as the temperature decreases, the resistance increases. The powertrain control module (PCM) supplies a 5-volt reference signal to the sensor on the TFT sensor signal circuit and measures the voltage drop in the circuit. When the transmission fluid is cold, the sensor resistance is high and the PCM detects high signal voltage. As the fluid temperature warms to a normal operating temperature, the resistance becomes less and the signal voltage decreases. The PCM uses this information

redvett · February 18, 2008

The GMT800 has only one CTS 2 wire and it sends a signal to the PCM & cluster. You must have a bad (test it in a boiling pot of water) thermostat. It must make 195F if you used a cooler thermo thats your problem or its just cheap or bad. The low coolant temp sets a SES and the trans only a high temp will set a SES. Make sure the thermostat is working by testing and make sure your cooling system has no air in the system.

FREAKENHYE · February 19, 2008

The GMT800 has only one CTS 2 wire and it sends a signal to the PCM & cluster. You must have a bad (test it in a boiling pot of water) thermostat. It must make 195F if you used a cooler thermo thats your problem or its just cheap or bad. The low coolant temp sets a SES and the trans only a high temp will set a SES. Make sure the thermostat is working by testing and make sure your cooling system has no air in the system.

Im with him on this one, air in system, and bad thermostat/lower temp thermo

sc4808 · February 20, 2008

Thanks for the advice guys, the replacement thermostat was a 195 degree Stant from NAPA, I may need to go back to the dealership and get an OEM replacement.

The air issue may have some merit, my coolant level was a little low about a month ago so I just topped it off (the "low coolant" warning came up in the DIC), maybe I have some trapped air, how would I go about purging it from the system?

Also, the local parts man (NAPA) is the one that told me that I have two CTS's on my truck, thats what his computer shows, I trust you guys but are we sure that the V6 4.3L only has one, if so that is more than likely the problem.

Once again thanks for the help.