P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
P0128 is a generic powertrain diagnostic trouble code indicating Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). It relates to the Fuel & Air Metering system.
DTC Breakdown (SAE J2012)
| System | Powertrain (P) |
| Type | Generic |
| Subsystem | Fuel & Air Metering |
| Fault Index | 28 |
| Standard | SAE J2012 / ISO 15031-6 |
What does diagnostic code P0128 mean?
The engine coolant temperature is below the thermostat regulating temperature, meaning the engine took too long to warm up to its proper operating temperature.
Severity Level & Driveability Details
Severity: Low to Moderate
It won't strand you, but it keeps the engine in open-loop mode, leading to poor fuel economy, high emissions, and lack of cabin heat.
Common Symptoms of P0128
- Temperature gauge stays low or fails to reach center.
- Heater blows lukewarm or cold air.
- Noticeably poor fuel economy as the engine runs cold.
- Check engine light illuminated due to low coolant temperature.
Typical Causes for P0128
- Thermostat stuck open (primary cause).
- Failed Engine Coolant Temperature (ECT) sensor.
- Low engine coolant level or air pocket in cooling system.
Diagnostic and Troubleshooting Steps
Monitor ECT sensor readings during a test drive. If the temp stays below 160°F-170°F (normal is 195°F-210°F), and the radiator hoses warm up slowly from start-up, the thermostat is stuck open.
Typical Repairs and Fixes
- Replacing the engine thermostat and housing.
- Topping off engine coolant.
- Replacing a failed ECT sensor.
Sensor Location & Parts Involved
The thermostat is located in a housing where the upper or lower radiator hose connects to the engine block.
Clearing Code & Recurrence
Clear the code with an OBD2 scanner and drive the vehicle to verify it reaches operating temperature.
Emissions & Smog Inspection Impact
Increases fuel enrichment, resulting in elevated tailpipe hydrocarbons and carbon monoxide emissions.
Free Check Engine Code Cheat Sheet (PDF)
Got your answer? Keep the top 50 most common diagnostic codes, their meanings, severity levels, and quick fixes handy in your vehicle's glovebox.
Sources & References
This diagnostic guide is compiled and verified against the following authority sources:
- EPA OBD II Compliance standards
- SAE J2012 reference guide
Frequently Asked Questions about P0128
What does the engine coolant temperature thermostat code P0128 mean?
The engine control module logged P0128 because the engine coolant temperature failed to reach the required operating temperature within a specified time.
Is it safe to drive with the engine coolant temperature thermostat code P0128 active?
Yes, but fuel consumption will increase.
How much does it cost to repair a engine coolant temperature thermostat code P0128 fault?
Thermostat replacement is relatively inexpensive. Parts are $30-$80; labor is $100-$200.
What are the common causes of the engine coolant temperature thermostat code P0128?
Trigger conditions for the engine coolant temperature thermostat code P0128 include: Thermostat stuck open, allowing coolant to circulate constantly (primary cause)., Failing Engine Coolant Temperature (ECT) sensor reporting low values., Faulty coolant level sensor or low coolant level..
Can I clear the check engine light for engine coolant temperature thermostat code P0128?
Clear the code with an OBD2 scanner and drive the vehicle to verify it reaches operating temperature.
Will the engine coolant temperature thermostat code P0128 cause an inspection failure?
Increases fuel enrichment, resulting in elevated tailpipe hydrocarbons and carbon monoxide emissions.