P0128 on Jeep
Coolant Thermostat Below Regulating Temperature
P0128 on Jeep vehicles indicates coolant thermostat below regulating temperature. Au7o has documented this code across 4 Jeep models — most commonly on Commander, Compass, Liberty. 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 Jeep range from $60 to $2,500, 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 Jeep by Model
Jeep Commander(1 issue)
- Engine Overheating / Cooling System Failures2006-2010
The Commander is susceptible to engine overheating caused by failing water pumps, cracked radiator tanks, leaking heater core connections, and thermostat failures. The radiator end tanks (plastic) are prone to cracking at the seam where they meet the aluminum core. Power steering cooler line leaks can also compound cooling issues. The 4.7L V8 is especially vulnerable.
Jeep Compass(1 issue)
- Thermostat Housing Coolant Leak2007-2017
The plastic thermostat housing on the 2.0L and 2.4L World engines is prone to cracking and leaking coolant. The housing warps from heat cycling, causing the O-ring seal to fail. This leads to external coolant leaks, overheating, and potential engine damage if not addressed. The same housing is used across Compass, Patriot, and Dodge Caliber platforms.
Jeep Liberty(1 issue)
- 3.7L V6 Exhaust Manifold Crack / Valve Cover Oil Leaks2002-2012
The 3.7L PowerTech V6 engine suffers from two common issues: cracked exhaust manifolds (causing ticking noise and exhaust leak) and valve cover gasket oil leaks. The cast iron exhaust manifolds develop stress cracks due to thermal cycling, with the passenger side being more common. TSB 09-001-10 addressed valve cover oil leaks. Head gasket failures have also been reported on higher-mileage engines.
Jeep Patriot(2 issues)
- Thermostat Housing Coolant Leak / Cracking2007-2017
The plastic thermostat housing on the 2.0L and 2.4L World engines cracks from heat cycling, causing coolant leaks and potential engine overheating. This is the same design flaw shared with the Jeep Compass and Dodge Caliber that use the same engine family. The plastic becomes brittle over time and cracks at the O-ring seal area.
- Engine Overheating / Water Pump and Radiator Fan Failure2007-2017
Beyond the thermostat housing issue, the Patriot suffers from additional cooling system failures including water pump failure, radiator fan motor burnout, and radiator cap failure. The water pump can develop an internal leak that mixes coolant into the engine oil, causing severe engine damage. The radiator fan may stop working intermittently, leading to overheating in slow traffic or idle.
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View P0128 across all makes →Frequently Asked Questions
What does P0128 mean on Jeep?▼
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 Jeep specifically, this code is documented across 4 models.
What causes P0128 on Jeep vehicles?▼
Common causes on Jeep: 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 Jeep?▼
Repair costs on Jeep range from $60 to $2,500, depending on the specific model and root cause.
Which Jeep models have P0128 documented?▼
Au7o has documented P0128 on 4 Jeep models: Commander, Compass, Liberty, Patriot.