2001 Dodge Ram 4x4, the heater suddenly stopped working. Hoses checked, fluid full, heater core is good...there are no leaks. Looked under dash to verify doors are moving. All working when moving temp from cold to heat and back. No hesitations. All fan levels work (1-4). There is simply no heat being emitted. What next?

You could have a leak coming from the intake or even the heads. Before digging into those difficult items, I'd check all of your freeze plugs for damage/wear/rust/leaks.

That sounds like a bad heater core talking

You have a blown heater core my friend. I have a quick fix and a permanent one. Quick fix is to unhook the hear lines from the fire wall and couple them together. That will stop it from over heating. 2 requires a friend and a full 6 hours. Remove the dash. I recommend getting a haynes manual for the dash removal. Hope this will point you in the right direction.

Sounds like the thermostat is stuck open or fans running all the time (if electric). If water pump was shot, then engine would get hot, depending on the temperature where you are.
Electric fans, or belt powered? If electric, do fans run all the time? If so, replace the fan relay module, which are prone to failure in Chrysler products.

are heater inlet outlet mix valves standard for all dodge heater cores. I have 2001 dodge cummings blows warm air not hot when gauge is 195 degrees would there be a mix valve under the dash because it is not inline outside the wall

low radiator coolant level OR bad thermostat and Lord I hope not,a leaky heater core.Check fluid level first the look at thermostat replacement only after the level check. Good luck and bless you in advance if you should catch a cold...from the cold!

give the stop leak ago. If it DOES work consider it a cheap quick fix. i wouldn't expect it to be a permanent fix though.. it may stop the leak in one spot but shortly after another one could develop at another part of the heater core. Or you could check the hoses and make sure its not them itself around where the hose clamps are. but if your 100% sure it is the heater core itself the best way to go would be replacement.

If the air deflector is moving correctly, Make sure your thermostat is not hanging open and that your coolant level is up to standards. While the engine is at running temp. check the two hoses that go through the firewall to the heater core. If they are both hot than one of the servos, vacuum or motorized may not be moving the deflector doors correctly. Try to hear any movement when you switch back and forth between defrost and floor heat.I doubt you need to replace the heater because of the age of your vehicle. Try to watch for any movement under the dash while you move the control. Good luck, Dana

Heater core

The first thing you have to do is make sure you have antifreeze circulating through you heater core. To do this turn your truck on and turn on the heater full blast,( not just the fan but your heat control valve ). Next, after 15 or 20 minutes, by that time the engine should be warm put your hand on the hoses going to your heater core. If you have circulation they will both be hot. If you do, means your heater core is not pluged and the problem is with your heat direction doors. This is usually caused from lost vacuum. You may have a vacuum hose off somewhere. Most likely, but not always under your dash, usually in some god forsaken place to get at. Start your vehicle and listen for leaks in vacuum.
Now. After checking the heater core for circulation you find that one hose is hot and the other is not that will mean the problem is in your heater core. Has there ever been any stop leak put in your radiator? This very well could be the sorce of your problem. If you are finding that one hose is hot and the other is not. Let the engine completly cool, several hours. Cold...Now remove the heater hoses from the heater core, Taking care not to make a spill (I know you will it's quite messy) antifreeze is POISENOUS to pets! Take a garden hose and flush your heater core out, both directions several times even after you get a good flow. NOTE: you will not need to use full water pressure to do this and I reccomend you don't use full water pressure because the water pressure in most cities is too strong for your cooling system components.
3rdly: If you do not have any circulation in your heater core. Both hoses are cold. Check the radiator hoses if bottom one is hot and the top one cold it means your thermostat is stuck shut and is the culprit. 3 out of 4 times this will be your problem. Usually the easiest thing to fix also.

NOW KEEP IN MIND: The first thing you must check is your coolant level. Your engine and your heater will not operate properly if this is low.

One more thing I would like to mention. You did not say how your engine heat was. You know, what did the temperture gage read. Was it hot? Was it cold after taking so long warming up and no heat coming out. If your emgine temperture is not up to normal operating temp, and you are not getting any heat. That could be because your thermostat is stuck in the open position, and the thermostat needs replacing.

I hope this helps you. If so please post and let me know.
Thanks!