Hey i have a 2003 Nissan Sentra 1.8L with 100000 miles on it and just recently the heater began to blow cold air most of the time and on some random occasions it would blow warm. i replaced the thermostat thinking that might be the reason but it wasn't. i have a good amount of coolant in it as well. any suggestions?

This system has to be purged of all air. This has to be done at a shop that has this tool. It basically vacuums out all the atmosphere just like an HVAC vacuum pump. It then sucks in the coolant thus no air pockects.

remove both hoses from engine that passes through firewall to heater. put a stiff piece of wire in the hole and make sure it is clear. If there is no restriction, then the heater is choked up

Might try a reverse flush of the cooling system. Might reverse flush the heater core too.

I recently tried the DIY service and repair manuals from a website and it actually helped me repair transmission problem in my jcb 3cx site-master. It would'nt move forward or reverse, engine was good both solenoids for forward and reverse worked, but still couldn't diagnose the problem.
I don't have much technical knowledge but I just followed it step-wise, and now it works perfect! covers wiring, codes, diagrams, torque specs, PCV, everything
got it from www.reliable-store.com

Are you talking about the engine cooling system or a problem with your Auto A/C?

You should check out www.reliable-store.com they have info on repair, service, wiring diagram, electrical diagram, fuse diagram, torque specs, troubleshooting,
PCV, gear box, owner's/service/repair/parts manual, fault codes, error codes, etc everything on all vehicles. must check it out

you hit the nail on the head ,cold and silicone chips ,they do not like the cold weather they can be temperamental to say the least ,as for finding the faulty component you will need an identical car that works ok ,then when yours will not start try swopping bits till you hit the one causing the problem.or sell it for a nice manual diesel version,

check the heating core or radiator for clogging or leaks

try unplugging the mass air flow sensor. it's between the air filter and the engine throttle chamber. it has an electrical connector. if it idles fine, you need a new MAF sensor.

My 05 sentra with 79k miles heat will warm up and then get cold after a mile of driving, we have tried to get the air pocket out, the mechanic stated he thought it could be a blown head gasket, another mechanic said it was a bad heater core. Any advice would be appreciated.