Hey everyone, so I had this same issue the past year or so. I have an 05’ Honda Accord Ex 4cyl Auto. Back when a heating system issue occurred, I was told that the heater control valve plate, located at the firewall, rusted from the swivel mechanism and potentially got lodged in the fins of the heater core. After 2-3 reverse flushes it appeared that there was no extraction of that broken piece from within the heater core. So the heater control valve was replaced, along with the water pump, as that was initially what we believed the problem was, and the thermostat was replaced as well. For more background, I replaced my water pump 5 days ago, this time sue to a wobbly pulley, new coolant, did the whole air bleeding process, so no issues there. Fast forward to yesterday, its cold weather season again and I wasnt going to deal with this lack of heat anymore. Finally went to another guy to get this flush done correctly… A 2008 Honda Pilot pulls out of his garage and he directs me to pull into the garage, I quickly explain that I have heat when revving up the engine or driving at highway speeds, but not at idle. I assure him the temp guage stays at normal temp all the time, coolant levels proper, and water pump recently done. So he asks me to pop the hood in order to observe the heater control valve movement when switch from HOT to COLD and back a few times. Then he says “Wait a sec, let me tighten this valve, it looks loose”… and in that instant, the car blew out HOT air without revving it. I was amazed and annoyed at how simple that was, how easily overlooked it went, but also how convinced I was I needed a flush or new heater core when the problem lay with the looseness of the heater control valve. Turns out that the Pilot that left just before me had the exact same issue, go figure! Hope this helps someone experiencing this issue with their Honda.