Why has the heater to my 2010 Toyota Sienna XLE started blowing cold air?

What is your question?

Heater to our 2010 Toyota Sienna XLE abruptly stopped working properly. Mileage: 128,400 Preventive Maintenance Performed Regularly: With the exception of the heater, the minivan is in great working order. The blowers work fine, but it now blows cold air most of the time, but will intermittently blow warm air for short bursts then blow cold air again. We have adjusted the temperature in the cabin to see if this makes a difference, and it does not. Radiator fluids are topped off where they should be. *Side Note: We have noticed the motor has also just begun to make a muffled purring baby diesel noise when running. - Not sure if this is part of the issue or not. Bottom Line. Doesn't work as it should anymore, and Maine Winters are COLD! Thanks for your time investigating this.

What makes this problem better or worse?

Unable to determine any factors contributing to issue

How long has your 2010 Toyota Sienna had this problem?

Few Weeks