Had P0128 obd, replaced t-stat, now no only cold air from heater. Heater core bottom hose warm top hose cold. Need help to get warmed up..

 

Very first thing check coolant level, then check for leaks.
Your heater working fine or not before you got the P0128 code?
Fully warm up your truck then use your scanner and check the ECT, what temperature you get now.
Is there a heater control valve on your truck?
Put your hand on the heater box (below passenger glove box) and feel it, now fully warm up then turn your heater to max and wait a few minutes, put your hand on the heater box again does it get warmer than before?
Do you have single or dual zone climate control system?
If you think you can handle to reverse the upper and lower hose before the firewall?

You should be more concern on your cooling system than your heater. Check 2 pdf below too.

 

My coolant temp gage was not coming off cold, replaced the twin set of T-stats, heat was fi e for a couple of days, but now it is just cold
heater was fine before changing T-stat's, now that the temp gage heats up the engine light has cleared, have dual climate controls

 

There are 2 hoses going to the heater core.if one is higher and the engine is cold, remove that upper hose to allow any trapped air in the heater core to exit the hose connection. Once the trapped air is out and you reinstall the hose, ,you can start the engine and the heater hoses should get warm as the engine heats up. and you should get plenty of heat. Don't forget to make sure the coolant level is up to the correct mark on the coolant reservoir. All this should be done when the engine is cold.

 

Hanky thank you, I will try this today and let you know how it turns out