I have a 02 Nissan sentra 1.8 heater is very weak even after it warms up I have cool heat not hot gauge always stays right in middle never over heats top radiator hose is hot and bottom hose cold same with heater core hoses if I drive car heat blows hot/ normal but if I sit car at idle cool heat I have a newer radiator and thermostat, I did notice if i Rev the engine few times to between 2/3 rpms parked heat gets hot and stays. And all heater hoses are hot, but after I drive the car a little car seems to loose a little hot heat at idle but stays warm but not hot at idle This all started when I noticed the heat was getting weak and I had a check engine light, that said thermostat / coolant temperature sensor code, thermostat has been replaced, but not coolant temperature sensor, I have tried burping the system many times and it does work all hoes get hot on car but next day or two back to very little heat again at idle but driving car I have hot heat , there is no leaks, car runs and drives great with no problems, I was told there is a bolt above the thermostat housing called air relief valve that is a bolt that you can use to get air out of coolant system, but never tried it out yet. Sorry for the long story but I don't know how to fix the problem. Any help would be appreciated
I would try running the vehicle on a slight incline (nose higher than tail) for at least a half hour at idle, with the radiator cap off. Make sure the heater temperature is set on the highest setting, and the fan medium.
You can also loosen that screw you mentioned on the top of the housing a few times, and let whatever air is trapped in the system out. The coolant system will be fully bled when nothing but a clean stream of coolant comes out - no bubbles, air, or sputtering.
It's possible the fault code was thrown because the temperature sensor was in a pocket of air, and was giving false readings. You should be able to look at the "pids" parameters on a decent OBDII scan tool to determine if the coolant temperature sensors make sense.
I'd start with a careful system bleed first to get ALL the air out, and eliminate that possibility.