Possible causes

– Leaking or stuck open thermostat
– Low engine coolant level
– Insufficient warm up time
– Faulty engine coolant temperature sensor
– Engine coolant temperature sensor harness is open or shorted
– Engine coolant temperature sensor circuit poor electrical connection

Tech notes

There is a service bulletin for the following Mazda models:

2004-2005 Mazda3 vehicles
2006-2007 Mazda5 vehicles
Mazda Factory Service Bulletin OBDII Code P0128

When is the code detected?

The engine coolant temperature does not reach to specified temperature even though the engine has run long enough.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0128 MAZDA Description

Indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.

MAZDA Technical Service Bulletins for P0128 code