P0128 on GMC
Coolant Thermostat Below Regulating Temperature
P0128 on GMC vehicles indicates coolant thermostat below regulating temperature. Au7o has documented this code across 3 GMC models — most commonly on Acadia, Jimmy, Sierra 3500HD. This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions. Typical repair costs on GMC range from $200 to $3,200, depending on the specific model and root cause.
Common Causes of P0128
- •Thermostat stuck open or opening too early
- •Faulty engine coolant temperature sensor
- •Low coolant level
- •Cooling fan running constantly (overcooling)
- •Wrong-temperature-rating thermostat installed
- •Corroded or loose coolant temperature sensor connector
- •Air pockets in the cooling system
- •Faulty PCM/ECM (rare)
P0128 on GMC by Model
GMC Acadia(1 issue)
- Electric Power Steering (EPAS) Failure and Loss of Assist2007-2016
The 2007-2016 Acadia uses an electric power steering rack that is prone to intermittent and permanent failure. The steering assist can fail suddenly with no warning, requiring significantly more physical effort to steer — a dangerous condition at highway speeds. NHTSA received over 800 complaints and opened an investigation. The failure mode involves the EPAS motor and control module overheating, producing a Service Power Steering warning. Failures are more common in warm climates and on vehicles that have been through multiple hot/cold cycles.
GMC Jimmy(1 issue)
- 4.3L Vortec Lower Intake Manifold Gasket Leak1996-2001
The 4.3L Vortec V6 uses a lower intake manifold gasket that develops coolant and oil leaks where it seals the intake to the block. The OEM gaskets use a plastic carrier that warps over time, allowing coolant to mix with oil or leak externally.
GMC Sierra 3500HD(1 issue)
- Duramax Water Pump Failure and Coolant Leak2007-2018
The factory water pump on 6.6L Duramax engines (LBZ, LMM, LML, L5P) is known to develop weep hole leaks and bearing failures, resulting in coolant loss and potential overheating. The plastic impeller on earlier units can also separate from the shaft, causing sudden loss of coolant circulation. GM issued TSB 07-06-02-002 addressing water pump replacement procedures for LBZ/LMM applications.
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View P0128 across all makes →Frequently Asked Questions
What does P0128 mean on GMC?▼
P0128 stands for "Coolant Thermostat Below Regulating Temperature." This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions. On GMC specifically, this code is documented across 3 models.
What causes P0128 on GMC vehicles?▼
Common causes on GMC: Thermostat stuck open or opening too early, Faulty engine coolant temperature sensor, Low coolant level, Cooling fan running constantly (overcooling), Wrong-temperature-rating thermostat installed. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0128 on a GMC?▼
Repair costs on GMC range from $200 to $3,200, depending on the specific model and root cause.
Which GMC models have P0128 documented?▼
Au7o has documented P0128 on 3 GMC models: Acadia, Jimmy, Sierra 3500HD.