2007 Chevrolet Colorado LT 3.7 2WD, P0128 Code

Just FYI, the Colorado pictured above is not the one this issue occured.  I did not take a picture of it, but I thought this one (another customer's) would do.

This vehicle came in with a customer complaint of the CEL being illuminated.  I ran the codes and discovered it had a P0128 DTC.  P0128 is a Coolant Thermostat code.  However, some tests need to be performed before relacing a thermostat.

I first performed tests on ECT and its wiring.  Everything checked out okay.

I then started the vehicle and let it idle for several minutes, long enough to reach normal operating temperature.  Below you will notice the temp hand has hardly moved even though the truck had been idling for several minutes.

I then connected my diagnostic tool to see what temp the sensor was showing.  Again, as with the temp hand, it was woefully low.  Sorry for the glare, but the temp is reading only 140 degrees. 

After taking it for a short drive, the temp remained very low.  It only got as high as 150 degrees.  Obviously, there is a problem in the coolant system not allowing the vehicle to reach normal operating temperature.

After gathering the evidence, I decided it was time to take a look at the thermostat.  To easily reach the thermostat on these vehicles, raise the vehicle and go through the driver's side wheel well.

After removing the left tire, the inner fender trim has to be removed. 

Here it is on the floor.  It has six retainers holding it on.

Now, the thermostat housing can be observed.

In order to drain the coolant on this vehicle, the lower radiator hose must be removed from the radiator.  There is no drain plug on these models.  It can be a bit messy, but not too difficult.  In the picture above, you can see the lower hose connected to the thermostat housing.

There are two 10 mm. bolts holding the housing on.  Below is the entire assembly (lower hose still attached).  On these models the thermostat, housing, and gaskets come as one assembly.

Upon inspecting the thermostat it was obvious what the problem was.  Notice below (I'm pointing to it) that the gasket has come out of place.

This was allowing coolant to bypass through the opening.  As a result, the thermostat was partially open and not allowing pressure to build in the system or allow the coolant to reach normal operating temp.

Once the new one was installed and the coolant was at full, the temp hand at idle came up as it is suppose to do.  The difference in temp can be seen below on the scan data.

Now the temp is reading around 200 degrees.  This is where it should be based on the amount of time it was idling. 

A quick test drive and check for leaks and a recheck of coolant level (after the vehicle had cooled) and the repair was verified.