I have the same van and had the same problem. The error code PO128 is an error code letting you know you need to specifically replace the Coolant Temperature Sensor. The Sensor is about $14.00- $15.00. Just a little work, not bad. I did it and actually helped my gas mileage also. I would recommend Advanced Auto Parts.
thermostat
P0128- coolant thermostat ( coolant temp below regulated temp)
Sounds like you need to replace the thermostat.
T-STAT
2004 gmc envoy with code p0128
PO128 refers to the thermostat not operating properly. You could replace the thermostat with a good quality one such as Stant. The computer will have to be cleared. If the code does come back then you need to have a repair shop check it out for you. Code PO128 is engine temperature below specs. You can try changing the thermostat and have the code cleared. If that does not work then you need to pay a shop to further diagnosis it. jd
For 1996 and newer Trouble code P0128 means:Coolant Thermostat (coolant temperature below thermostat regulatin temperature)
Either you have a bad temp sensor or your thermostat is staying wide open and letting the engine run to cool.
P0128 = Coolant temp below thermostat regulating temp. Replace the thermostat, the old one is stuck open.
First check your coolant level,then change your "ect" engine coolant temp sensor.If that fails change your therostat and coolant.
BDTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature " I have seen this several times this week alone. All three times I replaced the coolant temp sensor and it fixed the customers problem. john f.