Originally Posted by
foama
There is a separate thread on that!
Summarized:
With mostly 2014/2015 cars, heater core and coolant react to produce a gummy, sticky goo that clogs the core.
Prevention is to thoroughly flush the old coolant out and put the correct new coolant in the proper concentration back in before the core gets clogged.
If already affected, some have put diluted ammonia water (spirits of ammonia) into the core (not the engine!) overnight, and then force-flushed the core with a garden hose in both directions. Helps well and is cheap!
Others have replace the core ($$$!).
It pays to monitor PH of the coolant yearly. The PH should be around 8.5 Any change and the coolant is exhausted and should be replaced asap.