Drove home last night with heat. Woke up this morning with zero heat. Line going to h/core is nice and warm, hose coming out cold. 99xj 4.0. That makes 3 vehicles, all poor to no heat. Any ideas where to start when I get home? Thanks in advance.

Flush the core.

Core is 3 weeks old. I feel like whatever sends a signal for the valve to open and let water pass through core isn't working. I didn't slowly loose my heat. It just disappeared over night. The previous core wasn't leaking, just extremely poor heat. The new core only made it acceptable still not hot but now nothing. New thermostat and housing and temp sensor for housing. The engine runs at the same temp pre and post thermostat and housing. I broke sensor that is why it was replaced.

There is no valve on a 99. If one’s hot and one’s cold, it’s plugged. It doesn’t take much.
Did you flush the block and radiator?

Signal for the valve to open? The only "valve" in the cooling system for a 99 is the thermostat. If youre thinking of a heater control valve, the 97-01 did not come with one. If someone added one, delete it and toss it into the trash where it belongs. I doubt the new core needs flushed already. A plugged hose? Possibly. My guess is the blend door in the case. Your 99 uses an electronically operated blend door, which do go bad. That could be the original problem, and was disturbed just enough replacing the new core that it opened slightly more than it was. But, thats just a guess.

*NVM about the blend door. I misread where you said the hose is cold.

Found system a little low of fluid. Hopefully all the air is out now. Doesn't explain it all in my mind. Heat is OK at best. When core was put in coolant was clean no stop leak stuff in it. I do have the heat back, not sure what happened. Working OT again tonight. look at some more this weekend. Thanks for response. appreciate it.