P0128 on a GMC: Causes, Fixes & Repair Cost
🟠 Moderate severity · drivable
Safe to drive, but the engine running cool hurts fuel economy and heater performance, so plan the thermostat repair soon.
What P0128 means on a GMC
On a GMC, P0128 means the engine computer has detected that the engine isn't reaching its normal operating temperature in the expected time. On many GMC engines the coolant temperature sensor mounts in the thermostat housing, so thermostat trouble often comes with a sensor that needs replacing too.
Is P0128 common on GMCs?
P0128 appears on GMC trucks and SUVs like the Sierra, Terrain, Acadia, and Yukon as they age. A stuck-open thermostat is the usual cause, and replacing the housing assembly is a common fix on these engines.
Commonly reported on: Sierra, Terrain, Acadia, Yukon.
Symptoms
- Check engine light is on
- Temperature gauge reads low
- Heater blows lukewarm air
- Reduced fuel economy
Common causes
- most common: Thermostat stuck open
- common: Coolant temperature sensor failing
- common: Low coolant level
- less common: Thermostat housing leak
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Top up the coolant if it's low | Yes | $10–30 |
| Replace the thermostat | Yes | $40–120 |
| Replace the thermostat housing and sensor assembly | Yes | $80–180 |
Frequently asked questions
Other common GMC codes
P0128 on other brands
Looking for the full (non-brand-specific) guide? See the P0128 code page →
Browse all GMC codes: GMC hub →
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.