berto has an 01 lb7, he also started another thread about the same problem, possibly hopeing to get a faster answer
berto sent his truck to the shop for a coupl other reasons and came out needing injectors, and not fixing his heater core issue, possible that coolant isnt geeting to the core.....idk
berto do some homework, others will surely chime in on this, and always place as much info about your truck so we can help to make educated guesses, afterall we cant see or touch or hear your truck.....