I have a problem with cold air from heat vents.

The most common reason is the coolant is low.
Check coolant level when vehicle is cool.
If coolant is low start vehicle and fill radiator with coolant.
if coolant was needed high probability there is a leak
look at all cooling system hoses, thermostat housing, all over the engine to try and find leak and repair.

if coolant isnt low, make sure its not really dirty...reddish mud colored. If it is the radiator is probably clogged and needs to be flushed thoroughly..then refilled with coolant.

there will be two heater hoses coming out of firewall..they are connected to the heater core and one hose directs flow of coolant to the heater core, other hose directs flow away from heater core..start vehicle and run until engine is warm then feel each heater hose as close to the firewall as possible..if one hose is hot and other is cool..the heater core is clogged and will probably require replacement. also look in passenger floorboard if coolant is leaking there..heater core requires replacement.

if you check these things and find none of the issues, let me know, and I will tell you what to check next.
Good Luck with it and if you have any.question let me know

The bypass valve may be stuck open. Try working it manually to see if it is sticking . Valves are usually vacuum or electric . If vacuum actuated, check vacuum lines for cracks or holes. If electric, make sure wiring is intact and properly connected. You may be able to disconnect acuator and manually open valve. Valve may need to be replaced also.
Hope this helps.
Bob

If you flushed the coolant with the thermostat in, chances are you didn't flush the coolant. Regardless, it's easiest to check the controls inside first. Does the blower fan work? Does the air flow change from floor to vent, to dash as it's supposed to? Does the A/C change temps when knob is turned from cold to hot? Post results.

Thermostat.? if you haven't? Do it first then try it. While the car is running for a while touch the heater hoses that lead to the firewall. if one is cold and one is hot? You have a blocked Heater core. Follow the hoses back to the engine. Unconnect them both. By using a garden hose flush out the heater core do one side and then reverse by doing the other hose Lots of junk will come out so be care ful. Mistake people make is "Flushing the system" That doesn't mean your flushing the heater core.
Seeing that you have done everything else it could be the thermostat which you didn't list or the heater core being clogged. When you finished installing the thermostat and flushing the heater core out. Replenish the coolant system . If you have a bleeder on the thermostat housing? use it . loosen it and top off the fluid until you see fluid comeout of the bleeder. The system being air bound would cause no heat..

A low coolant level will do this as well as a thermostat being stuck in the open position cure is to replace the thermostat another cause is low coolant flow through the heater core and could be partially plugged flushing the heater core will fix this condition hope this helps

first check to see if your heater hoses are getting warm you may just have a bad thermostat, if you've had any recent work done to the cooling system you may have an airlock which does not allow any coolant too flow through the heater core. do you smell antifeeze ?

Sounds like you have air trapped in the heater core when you flushed the heater core. loosen the upper most heater core hose and run the engine untill coolant seeps out. Wait until the coolant has reached running temperature.

once truck is warm check both heater core hoses to make sure they are both warm if not then check the cold water valve could be stuck shut not allowing coolant to travel in to heater core.

There are 3 possible cases: 1- The Flap Divert air direction valve for air flowing through the heater core is not doing its job. 2- The heater core gets clogged up. 3- The valve to control amount of hot coolant flow to heater core gets stuck. Let's check if the water can flow to the heater core first, by disconnect the output hose from heater core while running the engine with heater set up at maximum level to see if coolant flowing out. Catch this coolant with a container to refill it back. This way you can eliminate # 2 and # 3. Good luck.

squeeze the hoses to get any air bubbles out and refill coolant as necesary. are you sure you put thermostat in the right way? may be backwards. also may be your sensor.