P0128 on RAM
Coolant Thermostat Below Regulating Temperature
P0128 on RAM vehicles indicates coolant thermostat below regulating temperature. Au7o has documented this code across 2 RAM models — most commonly on 1500, 2500. 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 RAM range from $250 to $3,500, 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 RAM by Model
RAM 1500(1 issue)
- 4.7L PowerTech V8 Head Gasket Failure / Coolant Intrusion and Valve-Seat Drop After Overheat2002-2009
The 4.7L PowerTech V8 is intolerant of overheating and neglected coolant. Head gaskets leak combustion gases into the cooling system (air in coolant, coolant pushed from the overflow, steam/coolant smell from the exhaust on cold start). Once the aluminum heads warp or a corrosion channel opens between a water jacket and a cylinder, the machined-in valve seats can drop, causing catastrophic misfire and internal damage. A block/combustion-gas test in the coolant confirms it.
RAM 2500(1 issue)
- 6.7L Cummins Water Pump Coolant Leak (Fire Hazard Recall 17V562000)2013-2018
The concentric (belt-driven) water pump on 6.7L Cummins trucks can leak coolant and suffer bearing failure. NHTSA recall 17V562000 covered 2013-2017 Ram 2500/3500 and cab-chassis trucks whose water pump lacked a vent hole, allowing coolant to leak onto hot engine components and create a fire risk. Separately, owners report the pump bearing seizing and throwing the serpentine belt, one at just 32,000 miles while towing, leaving the truck without cooling or charging. Symptoms include coolant weeping from the weep hole, whining/grinding from the pump, and overheating.
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View P0128 across all makes →Frequently Asked Questions
What does P0128 mean on RAM?▼
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 RAM specifically, this code is documented across 2 models.
What causes P0128 on RAM vehicles?▼
Common causes on RAM: 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 RAM?▼
Repair costs on RAM range from $250 to $3,500, depending on the specific model and root cause.
Which RAM models have P0128 documented?▼
Au7o has documented P0128 on 2 RAM models: 1500, 2500.