p0128
Moderate
GMC · Trucks
Coolant Thermostat Control System
Quick Answer
Error code P0128 on your GMC Trucks means: Coolant Thermostat Control System. This is usually fixable — see the step-by-step guide below.
Affected Models
Likely Causes
How to Fix It
1
Monitor coolant temperature with scanner
2
Feel heater hose temperature after warm-up
3
Inspect coolant level and condition
4
Test coolant temperature sensor resistance
5
Replace thermostat if temperature control is poor
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Thermostat assembly
Coolant temperature sensor
Engine coolant
Gasket and O-rings
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