I have checked the valve on the fire wall that I have read in other posts about that goes bad but it is working properly I was wondering if there was any other known problems that would cause me to have no heat it only blows cold air. I have a 2010 Nissan titan Pro 4x.
Bubbles in your coolant that work to the heater core could also cause problems. I had terrible luck with multiple radiators cracking in my '04 (I think I replaced it at least 3 times), and almost every time the heater wouldn't work, it was because there was a small radiator crack causing coolant to leak and air bubbles to form in the line.
You could quickly check if your coolant is low. If you suspect air bubbles, you can try letting the engine idle and heater on full blast with the radiator cap open to let burp out any air bubbles that may be in the line and heater core. If you have any air in the lines, you'll see the bubbles forming at the neck of the radiator and the coolant level in the reservoir lowering as the bubbles work out.
Might not be the issue you're having, but the way I see it, it's one of the easiest to check.
