Coolant Thermostat Below Regulating Temperature
The DURAMAX-P0128 (Duramax) diesel fault code means: Coolant Thermostat Below Regulating Temperature. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- intermediate
- Estimated cost
- $150-$400 for both thermostats and gaskets, DIY-friendly
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View pilot availability →What Are the Symptoms of DURAMAX-P0128?
- Heater takes a long time to produce heat
- Coolant temperature gauge reads lower than normal
- Reduced fuel economy
- Check Engine Light with no driveability issues at first
What Causes DURAMAX-P0128? (Most Likely First)
- Thermostat stuck open (Duramax uses dual thermostats — both often fail together) Very Likely
- Coolant temperature sensor drift (low reading) Possible
- Extended idling in cold weather preventing warm-up Possible
Can I Still Drive with DURAMAX-P0128?
Yes, but fix soon. DURAMAX-P0128 is rated moderate severity. It is not an immediate stop-driving fault, but it will not clear itself — book the repair rather than ignoring it.
Common Fixes by Vehicle
What techs usually find when diagnosing DURAMAX-P0128 on specific platforms — tap a platform for the fix and the exact part:
2001-2016 GM 6.6L Duramax Moderate DIY
Coolant temp below thermostat regulating temperature - thermostat stuck open. Duramax uses a dual-thermostat setup; replace BOTH when doing the job. Use OEM AC Delco; aftermarket thermostats often fail within a year on these engines.
Labor: 1.5 hrsFrequently Asked Questions
What is P0128 on a Duramax?
Duramax code P0128 (Coolant Thermostat Below Regulating Temperature) is a moderate-severity diagnostic trouble code triggered by the engine control module when it detects a problem in the associated system. On Duramax engines, always replace both thermostats at the same time — they are cheap and sold in pairs.
Is a Duramax P0128 safe to drive?
Yes, you can drive, but fuel economy and emissions suffer and in cold climates a stuck-open thermostat can prevent the cabin heat from working properly.
How much does it cost to fix P0128 on a Duramax?
Thermostat set with gaskets $50-$120 in parts; $100-$300 in labor.
Mechanics Guide Step-by-step diagnosis
How Do You Diagnose DURAMAX-P0128?
Monitor ECT in scan tool during warm-up — should reach ~195°F within 10 minutes of normal driving
Compare ECT to intake air temperature after engine sits overnight; they should read within a few degrees
Replace both thermostats as a pair (OEM AC Delco 131-100 or equivalent)
After replacement, bleed cooling system through the bleed screw on the thermostat housing
Sources 1
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines P0128 as a generic OBD-II code)
P0128 on Other Platforms
The same code ID appears across other engines, vehicles, and equipment. Diagnostic flow varies by platform — see the matching breakdown:
Which Vehicles Commonly Get DURAMAX-P0128?
- 2001-2004 Chevy Silverado 2500/3500 HD 6.6L Duramax LB7
- 2001-2004 GMC Sierra 2500/3500 HD 6.6L Duramax LB7
- 2004.5-2005 Chevy Silverado 2500/3500 HD 6.6L Duramax LLY
- 2004.5-2005 GMC Sierra 2500/3500 HD 6.6L Duramax LLY
- 2006-2007 Chevy Silverado 2500/3500 HD 6.6L Duramax LBZ
- 2006-2007 GMC Sierra 2500/3500 HD 6.6L Duramax LBZ
- 2007.5-2010 Chevy Silverado 2500/3500 HD 6.6L Duramax LMM
- 2007.5-2010 GMC Sierra 2500/3500 HD 6.6L Duramax LMM
- 2011-2016 Chevy Silverado 2500/3500 HD 6.6L Duramax LML
- 2011-2016 GMC Sierra 2500/3500 HD 6.6L Duramax LML
- 2017+ Chevy Silverado 2500/3500 HD 6.6L Duramax L5P
- 2017+ GMC Sierra 2500/3500 HD 6.6L Duramax L5P
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