Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
The DURAMAX-P0128 (Duramax) diesel fault code means: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- intermediate
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What Are the Symptoms of DURAMAX-P0128?
- Check Engine Light on
- Temperature gauge reads lower than normal
- Heater slow to warm up
- Reduced fuel economy
What Causes DURAMAX-P0128? (Most Likely First)
- Thermostat stuck open or opening early High
- Coolant temperature sensor reading low Moderate
- Long idling in cold weather Low
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 does P0128 mean on a Duramax?
P0128 means Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). The engine is taking too long to reach, or is not holding, the temperature the thermostat should maintain.
Is P0128 a Duramax-specific code?
No. P0128 is a generic OBD-II code defined by SAE J2012, so it means the same thing on any vehicle that sets it. The exact conditions GM uses to set it on its diesels are in GM service information.
Mechanics Guide Step-by-step diagnosis
How Do You Diagnose DURAMAX-P0128?
Compare the coolant temperature reading with a known-good thermometer once the engine should be warm.
Check the warm-up time against the service specification.
Replace the thermostat if the engine does not reach regulating temperature.
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:
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