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Mazda 3 P0128: Coolant thermostat – below temperature


P0128 is a relatively common OBDII fault code. It is activated when your Mazda 3 coolant temperature does not reach the proper operating temperature.

Mazda 3 P0128

It is a generic powertrain code, meaning that regardless of who manufactured the vehicle, it will have the same meaning (1996+).

3 P0128 Symptoms

There are not many symptoms with P0128 unless it gets cold. These are the symptoms associated with this fault code:

  • Lack of heat– The biggest symptom of the P0128 fault code on the Mazda 3 will be a lack of heat. When the thermostat does not bring the engine up to proper operating temperature, it means that the water passing through the heater core is not as hot. This results in less/no heat.
  • Engine service soon– Other than the heater not working as well, the only other symptom of P0128 is usually the check engine light itself.

P0128 Causes: Mazda 3

P0128 Mazda 3

Fixing P0128 is usually a fairly easy thing to do, since 90% of the time it will be the thermostat that throws the code. These are the most common causes of P0128:

  • Thermostat stuck open– The most typical cause of the P0128 code is a stuck thermostat. When the thermostat is stuck, it allows coolant to flow through the engine at all times. When this occurs, the engine is “supercooled” and never reaches the proper operating temperature. How to replace a thermostat (video). 3 Thermostat
  • Cabling– If the thermostat itself is not the problem, it will be the wiring/sensor associated with detecting the engine water temperature, or the coolant temperature sensor itself.

Mazda 3 P0128 Diagnostics

A good way to tell if it is a sensor or thermostat problem is to know if the engine is running at the proper temperature or not. It can be very difficult to know this in a vehicle that does not have a temperature gauge.

A good way to at least get an idea of ​​whether or not it is the thermostat or the sensor is to start the car cold and let it idle for about ten minutes. If you feel little or no heat from the heater, then it is a good indication that the thermostat is the problem and not a wiring problem.

Additionally, a mechanic will often check under the hood where the radiator hose joins the thermostat housing. If the hose is easy to pinch, water will normally flow through it. If you pinch the hose, make sure you account for the strap-on accessories and the fan. The fan can be turned on at any time.


P0128 is not a serious threat to the immediate drivability of your 3. Keep in mind that most modern injection engines like to run above 200 degrees. Letting them run too cold can cause problems after a long period of time. Good luck finding out what’s wrong with your three


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