2004 Chev Silv. Heater stuck on max hot after batt replacement

cab heat dead.
NO CAR TOLD AT ALL (MAKER, MODEL, YEAR)
you skipped test #1 find out what end of car is bay
engine side or cab side, cutting the job in half saving all that extra cash thrown.
testing 1,2,3.
hot engine, say drive 10miles/
I use my hand as a tool or my IR thermal gun.
if hand flies off too hot is GOOD temp, IR GUN is hands free.
To the thermostat housing and Rad hot end goes to 180f to 200F
yes then all that is good.
then the 2 tubes going through the fire wall. is next.
heat core cab tubes/hoses.
no water valve there, no need test it. Look for bypass hose here if you bought a car with no heat,(called looped, due to core leakage hack.)

the 2 tubes are at 180f is good one cold is bad.so core needs a backflush. if one is cool or luke warm.
ok both tubes hot the CAB side is bad, I just saved you big cash.
next we test the dampers, called blend doors.
if stuck closed the heater core air path is blocked so heat is gone.
make sure recirc door is active (no fresh air ) makes max heat.
top cause bad damper actuator. motor,, no car told so not possible tell more. now. 100 years of heater, I can not guess.

temp door is bad and needs replaced you can just see door motors under dash by gas pedal

What do you mean it "doesnt work", the DLC doesnt do anything. Its the port that lest the scanner communicate with the computers in the truck. The LIGHTER fuse supplies 12V to the power pin on the DLC. If the fuse is good, take it back and make them fix it.

YOUR actuators for blend door are stuck unhook battery let sit a few minutes and hook back up this causes hvac actuators to do a relearn and will sometimes free a stuck one

heater unit blocked ,take the pipes off and flush it through with hose in both directions

try this, it may work ,may not. unhook the battery for a few minutes, then hook it back up.. now turn the key on, but dont start the truck for 2 minutes. now start the truck, it may reset????

Blower fan only works in highest (#5) setting. Problem started a wek ago.

check the bulb socket for corrosion

try unhooking battery for a few minutes this will cause actuators to do a full sweep and reset. if it is still stuck then will need to replace the actuator for blend door on that side

Have you checked your coolant? if coolant is low it can cause an air pocket in heater core and it wont pass out until rpm's increase by the extra flow of water pump. if it is full then might wanna try removing heater hoses and back flushing heater core with water might have a little trash in it restricting flow.