My 2000 Dodge Stratus SE heater does not blow very warm air. If it's below 30 degrees it is really bad. I've had the radiator flushed, the heater core cleaned, and something else done but no one seems to know what the issue is.

Sounds like a thermostat and/ or the system is low on coolant.
Its probably not going to reach 200 degrees when the outside air temp is below freezing, but you should get plenty of heat if the system is working correctly.

Right think the problem is the heater core is blocked with corrosion ,now remove the bottom hose from the rad and the heater hoses then flush every which way you can including through the heater core reversing direction then ,refit all the hoses fill with water only and see if heater works if so the remove the bottom hose again and drain ,then add a minium of 3 litres of antifreeze ,its a anti corrosion agent as well as antifreeze and without it the internal of the engine will corrode and the slurry will block the heater first then the radiator ,

I'd say you have a faulty thermostat, be it new or not. If your temperature gauge goes to cold as your engine is taking you down the highway, the thermostat is not doing its job. The temperature should never come down to below normal until engine is shut off...

See if your library has a Stratus shop manual and check the heating system chapter. You didn't say if the engine was reaching operating temperature or not. If the engine is warm but you are not, I would say the problem is in the heat exchanger somewhere. If your engine is cold, you can't really expect the heater to get warm, now can you. Then you have a different problem to chase.

Lack of water on your floor means your heater core is not leaking. It could still be plugged somewhere. The fact that you get some heat initially, but it turns cold when you turn the fan on, sounds like a lack of adequate warm water to the heater core to me. If the engine is warm, it could be a circulation problem of some sort.

where did you buy your thermostat? if you bought a fail-safe style thermostat it could have been no good before you even put it in. also is your tempeture gauge coming up to operating temp?? if it isnt your thermostat may be stuck in the open position allowing the coolant to circulate freely and not actually warm up. i would also check the lines going into the heater core. if one line is hot and the other line is cold than your heater core is probably in need of a flush or replacement.

just because the gauge seems normal doesnt mean the thermostat is good, its 5 years old,and may be opening to soon, i would start by having the t-stat replaced and while thats being done have the cooling system flushed including a flush off the heatercore. i ve worked on a couple of these vans with heating problems, most were low on coolant due to a slow waterpup leak, the others with half plugged heatercores, due to the lack of coolant flushes. hope this helps.

the flap in the air box may be broken or not ingageing all the way check for vac leaks and switch the air from defrost to vent and see if its working properly

you can change the thermostat yourself for under 20 dollars. if you dont know how go to an autozone and they will assist you.

The coolant thermostat needs replaced or you need a coolant flush. Heater cores (in the dashboard) often clog and stop warm coolant from circulating into the heater. A coolant flush would cure this. Flushes usually run from 70 to 90 dollars to have done at a service shop.

BLEND DOOR STUCK OR BROKEN. NEEDS TO REMOVE DASH AND HEATER/EVAP CORE BOS TO REPAIR REPLACE.