so I have been battling with this for a long time. back in Jan/Feb, I had the same check engine light and it would not pass inspection. always P0128 (correct me if I am wrong) is insufficient temp for the coolant system. Mostly resolved with a new thermostat. Correct?
anyways, the light came on about 2 weeks ago. my wife drives the car about 4 miles to work, so I was not worried too much because the car is only no more then 10 minutes at any given time. on the car cord reader I used, it have two entries for P0128. entry 1 said simply, P0128. the second entry said pending P0128.

last night I removed the coolant reserve tank and emptied and cleaned it. it was somewhat dirty and had what looked like 3 small deposits of sediment.
I refilled with water for now just to see what happens.
(this is a different question, but should the system pull from the reserve tank?

I left the radiator cap off, and turned on the car to observe the system heat up. eventually, 20-30 minutes later, the system seemed to be circulating the coolant and the coolant eventually got hot. the radiator fan never came on, but inside the car, the ac worked fine. it got cold and the heat worked fine, it blew hot air (not warmer air but hot air) during this process and watching the coolant get circulated, I sort of figured the thermostat did open because of the flow. I would slowly refill with water through the radiator cap until it would no longer "drain"

after I did this for about 45 minutes and then turned off the car. I used the car code reader about 10 minutes later and the reader had only one entry. still P0128 but no longer had the second entry with pending. I pressed the clear button on the code reader (which is supposed to clear the computer)
I used the code reader on my mazda 2 and it came back as 0, so I figured it did not store the P0128in the code reader.
Do you think I resolved my issue with slowly refilling through the radiator?
Is the coolant system supposed to pull from the reserve tank to prevent the coolant from getting "low"?
is there any thoughts or suggestions out there?

thanks
Ian

 

The coolant system is a sealed system, when the coolant gets hot it expands and overflows into the overflow bottle under pressure, when the engine cools down, the coolant contracts and draws coolant under vacuum out of the overflow bottle back into the cooling system. Some cars have the radiator cap ( pressure cap ) on the radiator, some have it part of the overflow bottle.

 

On my 2007 Mazda 5 when the P0128 code showed up, I replaced the thermostat. It hasn't come back ever since I replaced the thermostat a little more than a year and a half ago. It was a bit of a pain to replace because the area you have to work with is a bit tight.