P0128: Coolant Thermostat Below Regulating Temperature
130 vehicles · 25 makes · $20-$10,500 repair
Most Reported On
P0128 is an OBD-II diagnostic trouble code meaning “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. This code is most commonly reported on 1999-2010 Subaru Outback (2.5i, 2.5i Limited, 2.5i Premium, 2.5i Touring, Base, Limited, Premium, Touring), 2007-2012 Toyota Tundra (1794 Edition, Base, Limited, Platinum, SR, SR5, TRD Pro), and 1999-2011 Subaru Impreza (2.5 RS, 2.5 TS, Outback Sport, RS), plus 127 other vehicles, with repair costs ranging from $20 to $10,500.
Common Causes
Typical Repair Cost
Based on 158 documented vehicle-specific issues. Actual cost depends on root cause and vehicle.
Diagnostic Tools
To diagnose P0128, you'll need an OBD-II scanner. Here are our recommendations at every price point.
ANCEL AD310 Classic OBD-II Scanner
by ANCEL
A simple, affordable code reader that reads and clears check engine codes. Great for quick diagnostics on any OBD-II vehicle (1996+).
- Read & clear engine codes
- View freeze frame data
- I/M readiness status
- No batteries or app needed
BlueDriver Pro Bluetooth Scanner
by BlueDriver
Bluetooth OBD-II scanner with a free companion app. Provides enhanced diagnostics, smog readiness, and repair reports sourced from a database of verified fixes.
- Enhanced diagnostics (ABS, SRS, transmission)
- Repair Reports with verified fixes
- Smog check readiness
- Free app (iOS & Android)
LAUNCH CRP123X OBD-II Scanner
by LAUNCH
A professional-grade handheld scanner that reads all four major systems (engine, transmission, ABS, SRS) with live data streaming and graphing.
- Engine, transmission, ABS, SRS diagnostics
- Live data stream & graphing
- AutoVIN for vehicle identification
- Free lifetime updates via Wi-Fi
Autel MaxiCOM MK808S Diagnostic Tool
by Autel
Shop-level diagnostic tablet with bi-directional control, active tests, and full system coverage. Ideal for serious DIYers and small shops.
- All-system diagnostics (25+ modules)
- Bi-directional control & active tests
- Oil reset, EPB, BMS, TPMS, injector coding
- 7-inch touchscreen with Android OS
As an Amazon Associate, we earn from qualifying purchases. Prices are approximate and may vary.
Vehicles Affected (130)
Cooling System Problems and Engine Overheating
Premature Water Pump Failure and Coolant Leaks
Coolant Leak from Water Outlet Housing
P0128 — Coolant Below Thermostat Regulating Temperature from Thermostat Stuck Open
3.4L V6 (L32) intake and head gasket failure
Water Pump and Coolant Crossover Pipe Gasket Leaks (3.5L/3.9L)
Water Pump Failure / Coolant Leak
Lower Intake Manifold Gasket Coolant Leak
1.4T Coolant Leak at Water Outlet and Thermostat Housing
2.2L Ecotec Coolant Leak and Thermostat Housing Crack
4.2L I6 Coolant Leak from Throttle Body and Water Pump
Intake Manifold Gasket Leak (3.4L/3.8L V6)
Water Pump Shaft Seal Coolant Leak (3.6L V6)
2.2L/2.4L Head Gasket Failure
Head Gasket Coolant Leak and Overheating on 3.4L V6
Water Pump Failure / Weep-Hole Coolant Leak on 5.3L and 6.2L V8
Lower Intake Manifold Gasket Coolant Leak (Dex-Cool Erosion)
Water Pump Failure and Cooling Leaks
Chronic Overheating - Multiple Causes
Cooling System Failures (3.3L/3.8L V6)
Head Gasket Failure (Especially Turbo Models)
2.5L V6 Head Gasket Failure
3.6L Pentastar Engine Stalling and Head Gasket Failure
Turbocharger Failure (GT/Turbo Models)
3.5L V6 Head Gasket Failure and Overheating
3.5L V6 Coolant Crossover Tube Leak
Pentastar 3.6L Oil Filter Housing Leak
Cooling System Failures (3.3L/3.8L V6)
Timing Belt Failure - ALL Engines Are Interference
2.4L DOHC Head Gasket Failure
Engine Overheating - Thermostat Housing Debris
Engine Overheating - Multiple Causes
2.2L/2.5L Head Gasket Failure
Head Gasket Failure
2.2L/2.5L Head Gasket Failure
Heater Core Failure - Dashboard Removal Required
Electrical and Software Malfunctions
Gen 1 V10 Head Gasket Failure
Cooling system weakness — water pump, thermostat, and collapsing lower radiator hose
HEMI Water Pump Premature Failure / Coolant Leak
Thermostat Housing Leak and Overheating
Head Gasket Failure and Coolant Loss
1.5L EcoBoost Coolant Intrusion into Cylinders - Internal Engine Block Failure
2.3L EcoBoost Head Gasket Failure and Coolant Intrusion
1.5L EcoBoost Coolant Intrusion into Cylinders - Engine Block Porosity
3.5L Internal Water Pump Failure
2.0L EcoBoost Coolant Intrusion - Open-Deck Block Design Failure
2.0L EcoBoost Coolant Intrusion - Open-Deck Block Failure
1.6L EcoBoost Engine Overheating and Coolant Loss - Recall 14S11
EcoBoost Coolant Intrusion into Cylinders
Head Gasket Failure and Coolant Leakage
4.0L OHV cracked cylinder heads and blown head gaskets from overheating
1.0L EcoBoost Degas Hose Coolant Loss & "EcoBoom" Overheating Engine Failure
1.6L EcoBoost Intake-Valve Carbon Buildup (Direct Injection)
Thermostat Housing Coolant Leak
3.8L V6 Head Gasket Failure
2.0L SPI Head Gasket Failure
6.0L Powerstroke Head Bolt Failure and Head Gasket Blowout
3.0L Vulcan V6 Head Gasket Failure
3.8L V6 Head Gasket Failure
4.0L Cologne V6 Thermostat Housing Cracking and Coolant Leaks
4.6L Intake Manifold Cracking and Coolant Leak
Internal Radiator Transmission Cooler Fails — Coolant/ATF Cross-Contamination ('Strawberry Milkshake')
Plastic Thermostat Housing Cracks and Coolant Leaks on 2.3L Duratec
Electric Power Steering (EPAS) Failure and Loss of Assist
Duramax Water Pump Failure and Coolant Leak
4.3L Vortec Lower Intake Manifold Gasket Leak
Engine Overheat/Fail-Safe Mode from Thermostat and Coolant Temp Sensor
P0128 — Engine Never Reaches Operating Temp from a Stuck-Open Thermostat
Water Pump Failure and Coolant Leak
V6 Timing Belt and Water Pump Required Service (Interference Engine)
Navigation/Infotainment Hard Drive Failure (4th Gen)
Expensive Timing Belt and Water Pump Replacement Required
Drive Belt Tensioner Bearing Rattle
1.5T Turbo Engine Coolant Intrusion Into Cylinders / Head Gasket Degradation (Class Action)
8th-Gen 1.8L Cracked Engine Block — Coolant Leak and Overheating (R18 Casting Defect)
VTEC Solenoid Spool Valve Gasket Oil Leak
P0128 — Engine Not Reaching Operating Temp from Stuck-Open Thermostat
P0128 — Coolant Below Thermostat Regulating Temp from Stuck-Open Thermostat
Head Gasket Failure (Shared Excel Platform)
Head Gasket Failure (Mitsubishi G4DJ/G4EK)
Thermostat Housing Coolant Leak / Cracking
Engine Overheating / Water Pump and Radiator Fan Failure
Thermostat Housing Coolant Leak
3.7L V6 Exhaust Manifold Crack / Valve Cover Oil Leaks
Engine Overheating / Cooling System Failures
2.0L Turbo Coolant Leak from Y-Fitting, Reservoir & Coolant Inlet Tube Fasteners
Stuck-Open Thermostat Causing P0128, Poor Heat and Reduced Fuel Economy (3.6L Pentastar)
HECU Electrical Short and Engine Compartment Fire Risk
Front Power Seat Motor Overheating / Fire Risk
HECU Electrical Short / Fire Risk
Idle Stop & Go Oil Pump Overheating
Idle Stop & Go Oil Pump Overheating
Sticking Brake Calipers and Premature Wear
Turbocharger Coolant Line Leak
Thermostat Sticking and Overheating
Head gasket failure / chronic overheating (age and cooling neglect)
Water Pump and Coolant Crossover Pipe Leak (3.7L V6)
EGR Valve Carbon Buildup and Coolant Leak (SkyActiv 2.5L)
Water Pump and Plastic Thermostat Housing Failure
Thermostat Housing/Coolant Outlet Leak (N18/B48)
Electric Water Pump Failure and Overheating
Thermostat Housing Leak and Coolant Loss
Electric Water Pump and Thermostat Failure
Radiator Plastic End Tank Cracking and Coolant Leak
P0128 — Coolant Below Regulating Temp from Stuck Thermostat / Water Control Valve (QR25DE)
P0128 — Coolant Below Regulating Temperature from Thermostat Stuck Open
P0128 — Coolant Below Regulating Temperature from Thermostat Stuck Open
Head Gasket Failure
EJ253 Head Gasket Failure (External Leak)
EJ25 Head Gasket Failure
EJ25 Head Gasket Failure
Head Gasket Issues
Phase I EJ25D DOHC 2.5L Internal Head Gasket Failure
EJ25 Head Gasket Failure (External Leak)
Water Pump Bearing and Seal Failure
Inverter Coolant Pump Failure
Water Pump Leak and Failure (2GR-FE)
1NZ-FXE Head Gasket Failure and EGR Cooler Issues
B58 Engine Coolant Loss Without Visible Leak
Coolant Bypass Valve Failure
Water Pump Leak (2GR-FE V6)
G16E-GTS Head Gasket Seepage Under Hard Use
P0128 — Engine Runs Cold from Stuck-Open Thermostat
Water Pump Bearing/Seal Failure Causing Coolant Leak and Overheating
Head Gasket Failure (Supercharged Models)
3.4L 5VZ-FE Head Gasket Failure
22R-E Head Gasket Failure
Head Gasket Failure
Head Gasket Failure (5E-FE)
V6 Engine Water Pump Leak and Failure (Weep-Hole Coolant Loss)
Plastic coolant flange / 'blue' temperature sensor leaking at rear of head
VR6 Cooling System Failure
VR6 Cylinder Head Cracking
Water Pump Housing Crack and Coolant Leak
Water Pump and Thermostat Assembly Leak
Water Pump and Thermostat Housing Failure
Engine Coolant Temperature Sensor (G62) Failure
MK7 2.0T Water Pump Failure
Coolant Pipe Failure Under Intake Manifold
FAQ
What does P0128 mean?
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.
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?
Repair costs for P0128 range from $20 to $10,500, depending on the vehicle and root cause.
Which vehicles are affected by P0128?
Au7o has documented P0128 across 130 vehicle models from 25 manufacturers: BMW, Chevrolet, Chrysler, Dodge, Ford, GMC, Honda, Hyundai, Infiniti, Jeep, Kia, Lincoln, MINI, Mazda, Mercedes-Benz, Nissan, Opel, RAM, Ram, SEAT, Skoda, Subaru, Toyota, Volkswagen, Volvo.
Sources (12)
- ForumHead Gasket Failure in 2.2 liter models - Subaru Outback Forums
- mediaSubaru Head Gasket Problems Explained - All Wheel Drive Auto
- ForumEj25 sohc 2000 ob head gasket - Ultimate Subaru
- mediaSubaru Head Gasket Problems Crucial Years - Automotive Body Care
- ForumEJ251 Head Gasket Problems - RS25
- ForumHead Gasket Problems Explained - RS25
- ForumHead gasket problem fixed in 2001 2.5 RS - RS25
- mediaJalopnik: These Subaru Model Years Were The Worst Affected By Head Gasket Problems
- manufacturerFel-Pro: Solving Subaru 2.5L Head Gasket Problems
- mediaHiRide: Subaru Outback Head Gasket Leak Symptoms Causes & DIY Diagnosis
- mediaSubimods: How to Spot a Blown Head Gasket in Your Subaru
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.