Diagnostic Trouble Code P0128

P0128 “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)”


What is code P0128?

Code P0128 stands for “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)”. This diagnostic trouble code can show up when the engine coolant is unable to warm up adequately within a specified amount of time after the engine has been started. Therefore, it is important to have a qualified technician diagnose the specific problem that caused the code to be stored in your vehicle’s computer.

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What does code P0128 mean?

Code P0128 on a diagnostic scan of your vehicle tells a technician that there is something going on with your engine’s cooling system.

When your engine is running, the combustion process causes a good deal of heat to build up. So does and friction from all the moving parts. It is important that an engine reach a reasonable operating temperature – typically somewhere between 180-220 degrees Fahrenheit. But too much heat can cause damage to the engine. Therefore, a cooling system is in place to help the engine maintain temperatures within a given range.

The cooling system is made up of a series of passageways inside the engine block. Throug these passages, liquid coolant (also called “antifreeze” for its ability to withstand cold temperatures) is circulated. As it passes through your engine, the coolant absorbs some of the heat and takes it out through a hose to the radiator. There, the heat is released from the liquid coolant and dissipates into the atmosphere before the coolant returns to the engine to continue its cycle.

At the same time, a thermostat senses the temperature of the engine and serves to open and close the coolant passages as needed. A sensor on the engine also measures engine temperatures and relays that information to the ECM (Engine Control Module).

An engine that runs too hot can lead to overheating. One that runs too cool can run into problems with fuel economy and increased emissions. If the coolant is unable to warm up adequately within a specified amount of time, the thermostat cannot operate as it should to regulate engine temperature. When the ECM detects this problem, it stores code P0128 and turns on the check engine light.


What causes code P0128?

A number of causes can trigger a code P0128. That is why an accurate diagnosis is essential. Causes of this code include:

Most likely

Other possibilities

  • Air trapped in coolant passageways
  • Damaged or corroded sensor wiring or connector
  • Engine cooling fan stuck on high speed


What symptoms are associated with this code?

Common signs associated with a code P0128 include the following:


How does a technician diagnose code P0128?

If a diagnostic scan of your vehicle indicates a code P0128, a technician will perform tests to pinpoint the problem. These tests include checking for any related trouble codes. The technician will also use the scan tool to view live data and feedback (if available) from the thermostat and coolant temperature sensor to ensure that the data is consistent. If the coolant temperature is low, the technician might also check the coolant level or attempt to bleed the system of any trapped air. The wiring harness from the ECM to the sensor will also be inspected for cracks, corrosion, contamination, or bent pins on the connectors. It is also common to conduct tests of the thermostat and temperature sensor directly and an inspection of the cooling fan assembly.


How serious is this code?

Code P0128 is a common trouble code, but it is not among the more serious vehicle problems. Still, driving with this code can lead to a drop in fuel economy, a failed emissions test, and a lack of heat inside the vehicle on cool or cold days. Over time, increased engine wear can result. These problems persist over time. Therefore, it is recommended that you have your vehicle inspected and serviced as soon as it is reasonable to do so, typically within a couple of weeks.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.


Related Diagnostic Trouble Codes

P0118 Engine Coolant Temperature Sensor Circuit High

P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control

P0126 Insufficient Coolant Temperature for Stable Operation

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