My heater wasn’t working the last two days on my drive into work, it’s a short drive but it’s always heated up well before I get there. I’m wondering if it’s the blend door, but I’m a little perplexed because yesterday morning I pulled into work and checked the heater hoses to see if both were hot and one was hot. Then on my way home from work I stopped at the auto part store to grab a coupling to bypass the heater core in fear of something being stuck in there and not wanting it to release into the radiator, and the both hoses were hot, but then the hot air only lasted a little while and then when switching back and forth hot and cold it stopped working almost immediately. Is there a valve on the system that only opens the core for flow connected to the blend door being in the heat position, any information on how the system works would be helpful, I have changed the entire cooling system on my truck and did run water through the heater core a couple months ago and no chunks came out or anything raly