Thermostat MISHIMOTO or OEM Mazda

Buy an OEM thermostat from Mazda. The aftermarket ones are sometimes not identical.

I just had the code P0126 on my 2021 mazda cx-30 with 20,000 miles. Seems to be the same issue

when installing a mishimoto thermostat, check that the shoulder/flange is the same thickness as oem. Otherwise your stock hardware WILL NOT FIT. You will bottom out the bolt in the timing cover and snap that mf. Those bolts are extremely hard to access and a MAJOR HEADACHE to get extracted.

mishimoto changed the design of the thermostat when I purchased and you will need shorter bolts they are not provided

I got a bad mishimoto thermostat with similar results.

I bought and installed a Mishimoto thermostat that activates at 68°C, while my vehicle OEM (Original factory) thermostat activates at 88°C and fully opens at 100°C. With the Mishimoto thermostat (68°C), the coolant temperature basically stays 71 ~ 74°C when driving, from OBDII reading. Short Term Fuel Trim was OK, between -10% and +10%. I checked the spark plugs after one week, they were fine.

However, after three weeks (last Saturday to be precise), the Check Engine Light was on when I started the vehicle, and OBDII gave Code P0128. I searched the code online and got: coolant temperature fails to reach the proper operating temperature, or Coolant Temperature Below Thermostat Regulating Temperature. My thinking was the engine finally realized that the temperature was below the operating temperature, due to the Mishimoto thermostat.