I have a 2012 dodge journey crew. 157000 km dual climate. Both sides were blowing cold, driver side slightly warm after revving the engine. I got it flushed twice & both sides were hot for a week or two, the shop said the heater core needed to replaced. It was replaced yesterday & it was hot both sides. The passenger side took a bit to be the same temp as the driver side but eventually worked. Took it out of town today & the passenger side started blowing cold after an hour on the highway & isn't working anymore. The driver side still gets hot when parked or revving the engine but stays warm on the highway & stays hot while driving in the city. I changed the thermostat a year ago as well. I'm taking it back to the shop Monday but I'm wondering if anyone else has had this same issue.

 

Most likely there is an air pocket in the system. Heater cores can be stubborn to bleed out. all the bends and height different makes lots of holding spots for air, but over time ad pressure pulses they eventually "pop" or escape, creating a low coolant situation and then the coolant isn't always forced to the heater core, especially when the thermostat is open. Is the radiator and/or surge tank topped off to the full mark