2004 Chrysler Sebring Heat works intermitently

I am not a professional, but I have been repairing Mopars for a very long time.

You did not mention how the water temperature gauge was reading. I suspect that the engine water temperature indicator is reading strangely if you do have the following problem.

The control for selecting the various ports in the A/C and Heating is via engine vacuum. With the engine running, switch back and forth through the settings and verify that they are appropriately changing. I suspect, but I do not know for sure, that the water valve may be actuated via vacuum too (you should be able to detect resistance with a mechanical valve). Vacuum leaks may affect your engine performance too.

I think you may be experiencing a so-called air lock in the heater core due to a slow leak in the 'water outlet housing'. When you turn off the engine after the car is well warmed up, check under the hood on the right (passenger side). You may see a bit of steam, or a wet, bubbling on this housing. As the engine cools, it may be drawing air into the system at this point. The housing is located on the right side of the engine on top and has a large and two small hoses attached, plus a bleeder valve I would suggest you not mess with, and a water temperature sender connection.

I just had to replace this water outlet housing and temperature sensor (sending unit) on my 2004 Sebring 2.7l v-6. This wondrous part is actually a glued-together 2-piece plastic part which had developed a leak between the two pieces, and which is easily seen from above.

Wait until your engine is cool then read on. (Be aware that an encounter with hot coolant can be less fun than a herd of upset bumble bees).

Repair is easy; drain enough coolant from the radiator (lower radiator on the right side, drains straight down into your container) to allow the removal of the part. Anti-freeze can kill cats and dogs, so contain it! A nifty little flexible box-end ratcheting wrench (Westward part # 1LCN2 available through Grainger) for one of the attaching bolts, 8mm size costs about 21 bucks will be a BIG help (a local dealer wanted some $320 to do this job, the housing part was $140). Clean the mounting surfaces before you put the new part on, and caution, the so-called Dorman after-market part was not recommended by my parts store, who strongly urged me to buy the Mopar part). This took me less than 1/2 hour.

Make sure you replace the coolant, and then bring it up to the proper level, checking it frequently for several cycles of driving the car and letting it cool down. The low coolant level is the culprit in your case I would bet, so be prepared to add more than you drained. There are reported issues with messing with that bleed valve, so I suggest that when you're finished, you put the cabin temperature setting on hottest before you first start the engine, and that you pay attention to the repaired area for leaks. On the old stuff I have tinkered with for 35 years, I have not had an issue with an air lock in the heater core using this method. (But some folks may have).

I repeat, I am not an expert, so if you are not a DIYer, be aware that there are several ways this can hurt you or your wallet.