P0128 Nissan Altima
Coolant Thermostat Below Regulating Temperature · Powertrain Code · Medium · Repair: Easy
What is P0128?
The engine coolant is not reaching the expected temperature. Most commonly caused by a stuck-open thermostat, low coolant level, or a faulty coolant temperature sensor.
When the Nissan Altima triggers code P0128 (coolant thermostat below regulating temperature), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Nissan Altima.
This repair is rated easy, meaning basic tools, no lift needed.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Nissan Altima.
Common Causes on Nissan Altima
- Thermostat stuck open on 2.0L MR20DD or 2.5L QR25DE engine
- Coolant temperature sensor providing incorrect readings to ECM
- Thermostat gasket degraded from extended coolant service intervals
- Low coolant from slow leak at lower radiator hose clamp
Symptoms
- Engine takes abnormally long to reach operating temperature
- Heater blows lukewarm or cold air
- Temperature gauge stays below normal or never reaches middle
- Slightly reduced fuel economy from extended open-loop operation
- Check engine light illuminated
Notes for Nissan Altima
Altima and Sentra thermostat replacement is straightforward; located at the engine block coolant outlet.