Diagnostic Code P0128

[deyomatic]

Hey there. I let a friend borrow my '05 Sierra 4.3 for the week and on day 2 he called me to tell me that the check engine light came on so he used his father's OBDII tool and found a "P0128" code. It appears that means "Coolant Thermostat Below Thermostat Regulating Temperature." Does this mean I just need a new thermostat? Is this one sticking open or something? I'm a little worried because the temp needle has always read (what I think is) a little low but the C/E light never came on before.

 

I've changed several early model SBC and Ford thermostats. Is it in the same location on this 4.3? A cast iron elbow on the front of the intake? Any tips for refilling a cooling system that doesn't have a proper radiator cap, like this one?

 

Thanks.

[deyomatic]

No tips? Anyone?