DTC P0128 and No Heater

welcome to priuschat! could be clogged, have you had a flush and fill? maybe change out the thermostat. how's the water pump? serpentine belt? all the best!

 

Hearing fluid move from time to time, sounds like the engine cooling loop, in the radiator, was low to being with. You confirmed this by having to refill it. You thought the system was full, but it was not, as you still heard sloshing (fluid movement) sounds, after this top off. Air was still in the system before and after you refilled it. If all air was gone, you wouldn't hear any sloshing sounds. Additional air has been introduced into the engine cooling loop as you opened a hot and pressurized system.

Bentley states:
P0128 = Coolant Thermostat (Coolant temperature below thermostat regulating temperature
Possible trouble areas:

• thermostat
• cooling system
• engine coolant temperature sensor
• ECM

Since the engine cooling loop now has air in the system; unfortunately you did not wait long enough for the radiator/engine to cool before removing the radiator cap. Drain the engine cooling loop. Replace the thermostat. Test the old thermostat to see if this was the culprit. If the old thermostat opens at the designated temperature, then move down the trouble tree to the next item.

Was the inverter coolant ever changed? If no, change it out now, since the car is up, and you are already doing the engine loop.

You ever check the inverter coolant level? Do you see "turbulance" in the inverter coolant reservoir, when the car is IGnition-ON?

 

So, if there is air in the system, why do you just not "burp" the system and get all the air out? It is probably all that is required. Also the possibility exists you have a leak, have you checked the engine coolant pump for signs of a leak?

 

One common place is the engine coolant pump. Check around there for a pinkish residue. I only suggested a leak as a reason why there is air in your system. Do you know of any reason for there to be air to be in the system?

There are many threads on this subject by Patrick Wong (such as this one). There are also posts on how to get the air out of the CHS tank. If you use the search box at the top right of the page and search for change engine coolant posted by Patrick Wong and you will get plenty of reading on the subject.

It is possible that with air in the system and it not circulating properly that it could cause both problems, the heater one in particular. If you purge the system of air, you could then see what problems, if any, persist.

 

1. I suggest that you replace the radiator cap now, using the correct Toyota part. When the radiator is cold and you remove the radiator cap, you should not receive a surprise as you did.

2. I also suggest that you need to purge the air out of the engine coolant system. The cabin heater not working is indicative that coolant is not circulating adequately through that portion of the engine coolant system.

3. My post here explains how to replace the engine coolant pump and thermostat. An embedded post explains how to purge the air out, which is not simple and requires you to run the coolant heat recovery system pump. Since you have Mini VCI you can utilize the command to run that pump, if you like.
How to replace engine coolant pump and thermostat | PriusChat

4. After you are sure that no air remains in the engine coolant loop, then if you still have DTC P0128, you will need to replace the thermostat which apparently is stuck open. If you have a handheld instant read infrared thermometer, you can use that to measure the temperature of the radiator hoses and the cylinder head, to see what the temperature is. Normal operating temperature will be ~185 degrees F.