Coolant Thermostat (PO128)

It's already failed, that's why the P0128 DTC. With low coolant temperature the ECM won't switch to "Closed Loop Mode" therefor it's dumping fuel to the combustion chambers, it's kind of running a carbureted engine with the choke on, so your gas mileage will be pretty bad. It takes a little over 2 hours to replace it. There are different quality parts are available, just call around. You will also need coolant. Zee