I have a 2001dodge caravan that I can’t seem to get the engine temp up on. I’ve replaced the thermostat and the coolant temp sensor and topped off the coolant. It raised the heat to the first line but once the vehicle is being driven the temp drops to 0 and heater blows cold. During the summer the vehicle runs at normal temp. Have disconnected the ambient temp sensor which did nothing to better or worsen the problem. Help plz
Have you verified that the new thermostat is actually working? Sometimes brand new stuff is dead right out of the box.
+1 to shadowfax’s comment.
Additionally…when the OP topped-off the coolant, I hope that he did it via the radiator cap, and not via the overflow reservoir.
@Bird582
Was Air Purged From The Cooling System While It Was Being Refilled At The Radiator? There Is Probably A Bleeder Valve At The Apex Of The Cooling System. Did You Locate One And Did You Use It?
CSA
Did you put the thermostat in backwards? Its easy to do, I’ve done it myself. IF its in backwards the engine won’t hear up when its cold outside.
I hear this complaint often when the weather starts getting cold. The last 4 vehicles that I fixed for this involved bleeding all the air out of the cooling system. There will be little to no heat if the coolant is low and air is in the system. Topping off the coolant is a little more involved then just adding coolant to the radiator.
How are you determining the engine temperature? By the engine coolant temperature gauge on the dashboard? B/c there’s little to no heat coming from the passenger compartment heater? All the above ideas should be considered, but if the heater seems to be working normally, perhaps its just a gauge problem.
The engine coolant temperature – once the engine is warm – is determined entirely by the thermostat. If it is actually running too cold, either there’s something wrong with the thermostat, or the coolant level is incorrect or there’s air in the cooling system, either of which could adversely affect the thermostat function. I don’t think the various temperature sensors the engine uses affect the coolant temperature at all, they just measure it.
There is one exception though. If it is extremely cold, sometimes even a working thermostat won’t allow the engine to properly warm up. This never affected either my 60’s Ford Galaxy or my 70’s Ford truck. But when I lived in Colorado on really cold days some of my neighbors would put a piece of cardboard in front of their radiator to allow the engine to warm up.
@GeorgeSanJose
" . . . a piece of cardboard in front of their radiator to allow the engine to warm up."
Sounds like a poor man’s radiator shutter 
When I was in North Dakota we used to put a piece of cardboard or blanket in front of the radiator in winter. An engine that’s been sitting overnight at -40F and is started at -20F is not going to warm up normally… even if one plugs it in at night (I always did).
Not only that, the foam in the seats will be frozen solid… like marble. And the tires we had in those days would develop flat spots.
Lived in ND also, had the block heater and used it on a regular basis, no complaints about heat in the car, probably because I was bundled appropriately. Later in a warmer climate had a work vehicle with heated seats, what a difference! Don’t have heated seats in my car, but usually have the window open to expell cig smoke anyway.
I had a lower radiator hose heater. Worked great, but at sub zero temps the radiator dissipated more heat on the highway than the engine would create. Of course it WAS a little 2.3L aluminum engine…