P0128 on RAM: How to Diagnose & Fix
Coolant Thermostat Below Regulating Temperature
2 RAM models · 2 documented issues · 2 critical · $250-$3,500 repair
P0128 on RAM by model: symptoms, diagnosis and fix
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.
Common Symptoms
- Coolant leak / weeping from water pump weep hole
- Whining or grinding from the front of the engine
- Overheating
- Thrown or shredded serpentine belt
- Low coolant warning
- Sweet coolant smell
How to Fix
Check the VIN against recall 17V562000 (repair is free at a Ram dealer for affected trucks). For out-of-warranty failures, replace with the updated vented water pump (Cummins service PN 5473057 supersedes earlier numbers) and inspect the serpentine belt, tensioner, and fan hub while access is open. Address any coolant weeping promptly to avoid belt loss and overheating.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Mopar 68414058AB updated 6.7L water pump with seal
2013-2018 Ram 2500 6.7L VIN L as explicitly listed; supersedes 68414058AA and includes seals. Check T51 VIN eligibility before buying.
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Sources for this issue (3)
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.
Common Symptoms
- Coolant loss with no visible leak
- Air/bubbling in cooling system, coolant pushed from overflow
- White steam or coolant smell from tailpipe on cold start
- Overheating
- Rough idle / misfire after an overheat event
- Milky oil in severe cases
How to Fix
Do a combustion-leak (block) test to confirm. Fix requires head gasket replacement with resurfaced or new cylinder heads (dropped valve seats mean a rebuilt/new head, not just a gasket). Use a Mopar/Fel-Pro MLS head gasket set and always replace the thermostat, water pump and radiator, and flush the cooling system on schedule to prevent recurrence. Parts+labor typically $1,500-$3,500 depending on head damage.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Fel-Pro HS26157PT head gasket set
Do not use for 2004-2009.
- Fel-Pro HS26157PT1 head gasket set
2004-2007 Dodge Ram 1500 4.7L catalog branch, HS26157PT1; confirm VIN/valve-cover configuration.
- HS26157PT2-pattern 4.7L head gasket set
2008-2009 Dodge Ram 1500 4.7L within seller's 2008-2010 coverage; Replacement 598-237/interchange HS26157PT2. Fitment/availability: 81 shown in stock with add-to-cart live at review
- Fel-Pro ES71129 cylinder-head bolt set
Confirm VIN and bolt count/length before order.
- Zerex G-05 50/50 HOAT coolant, one gallon
G-05 HOAT chemistry for compatible 2002-2009 Chrysler applications; verify the owner's manual/current fill and do not mix incompatible coolant.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (2)
P0128 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0128 mean on a 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 it is documented across 2 models: 2500, 1500.
What are the most common causes of P0128?
The most common causes of P0128 are: 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). The specific cause varies by vehicle.
How much does it cost to fix P0128 on a RAM?
Published repair estimates on RAM range from $250 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0128 on a different make?
View P0128 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.