My 2018 was not getting much heat in the cab. Local shop did a full coolant flush, including heater core. Seemed to get a little warmer, but today it was either putting out hot air as it is supposed to, or cool air. Fluctuated back and forth a few times. I noticed that if the temp on dash was 198ish or higher, was nice and toasty in the cab. When temp dropped to190, it was just like having an outside air vent open. Blend door and actuator are working properly. Not low on coolant. Going to change out the thermostat, but would a faulty thermo. be the cause? In 30 plus years, I have never seen this to be the case, but newer technology...?

 

IIRC, these newer dodges have a problem with their heater door. Since I don't own anything older than 1992, this is just a blip on the memory screen.

 

UPDATE. New thermostat did not help. However, when the truck gets up to full operating temp, I get heat in the cab, as long as I am not moving or driving at slower speeds. At 55+, heat goes away. At a loss.

 

You didn't mention if any work involving the cooling system prior to the local shop doing a flush. Earlier CTDs had issues if you didn't 'burp' all the air out of the system but I don't know if it includes your year. One clue that stood out was the temp going to almost 200* then dropping back. Typical of trapped air.
Back to morning brew . . .

 

No other work on the truck before the flush a month ago. One thing I did not mention or think much of before all this, when the fan speed is on high, less heat than on low. Talked more with the shop and one of the mechanics said possible that the head gasket is cracked and wants to run a quick dye test. Aside from that, myself and the shop think the heater core, although has fluid moving thru it, is restricted.
Find out more next week.

 

Heater core was more than 50% plugged. Not sure how or why. I have never had add fluid. But with greatly reduced surface area, the old (original) core could not keep up.

 

I hope this solves your issue. It would be interesting to see what it was that plugged the 50% of the core.
Back to morning brew . . .