Heater not working

#1 Listen if you hear the blend door flap open and close if you do then working good.

#2 run car to normal temp and touch both heater hoses and make sure temp is similar if one is hot and the other warm then your heater core is clogged and most likely that can be the problem it build lime from having tap or well water in coolant system distilled water is recommended but who uses that.

#3 run car when cold with cap off @ 2k rpm and top off radiator. Then close cap and continue to rev @2k until normal temp if heater hoses are not equal in temp heater core clogged.

there is probably an air pocket in the system. if it has a bleeder valve , bleed the system while running or get a burp funnel

The thermostat may be stuck open or there may not be one in the thermostat housing.

have 2002 altima,blower works no heat at idle and very little heat when accelerating.I changed carbon cabin filter,made sure little gear under dash to temp knob wasnt broken,flushed and bled system.Nothing worked so checked hoses to heater core and found inlet hose was hot,outlet hose was not.So investigated this problem and came to conclusion heater core was clogged.An easy way to check is to get car to normal running temp. put on heater and blowers at full.Go under dash on passenger side where counsel meets firewall,this is where heater core core inlet and outlet aluminum tubbing go through firewall,you will be able to feel both tubes one closest to you is outlet, so if this one is not hot and and other is your heater core is clogged.I detached both inlet and outlet hoses to heater core from water control valve.Then I blasted water with waterhose and nozzle set at jet,first through outlet hose to blow gunk out,then through inlet hose until water runs clear.Reatach hoses fill and bled system.heater works great nice and hot!

If the temp is high at freeway speeds then the thermostat would be suspect. If the temp is high in slow traffic then suspect the cooling fans or its controls

check to see if both heater hoses are same temp when engine is at normal running temp if not heater core not flowing coolant may need to flush also make sure it does reach normal temp around 200

if you dont find any significant leaks .. then watch for steam out the exhaust after warm up .. or smell for antifreeze there soon after startup .. that would indicate a blown head gasket. it would have a damaged area between a water jacket passage and the combustion chamber of one cylinder ... if you can see inside the radiator you might see bubbles when running in this case .. the escaping exhaust gas (into the water jacket) can force water out of the block ... that could expain the fast heat up time .. if there is water in the block then it should take longer to heat even if other things are wrong .. a bad thermostat could cause overheating but would normall take a while before the temp reaches "peg" . same thing with a water pump if it had a damaged impeller. the only way the engine could get that hot that quickly is if there is little or no water in the water jacket .. a bad head gasket is the most likely reason for that .. of course make sure oil pressure is normal .. if the engine was not also losing water and obviously overheating then you could suspect a bad temperature sending unit ..but sounds like thats not your case .. be careful allowing the engine to get that hot .. it could warp the head causing more expense .. another frequent sign of a blown head gasket is the heater will quit working at idle but will start to work at higher RPMs... if you repair the gasket yourself, many engines require new head bolts .. you cant use the old ones over again .. the bolts are designed to stretch to accomodate expansion as things get hot .. special instructions will come with new bolts...

Thermostat regulates coolant temp so that the engine doesn't run too cool, It has no effect on the heater temp once the engine has reached operating temperature. Check that the hoses running from engine cooling system to the dashboard are not blocked, bent or broken. (Actually if broken, engine would lose all coolant fluid, and you would notice the mess). Check that the heater switch is doing its job, and actually activating the heater. All that said, If your car happens to have NO thermostat, engine will never reach a normal temp, and the heater will not work.

You may have an air pocket in your system which some times happens.  One way to get rid of it is to take the upper hose off on the engine side,  fill your system with water until it runs out from your engine block, replace the hose, start your car turn you heater on and set to hi keep the radiator cap loose until the temp start to rise then close tight.  Once the air pocket had been remover you can drain some of the water from the radiator using the lower drain plug and add antifreeze.   

you ether have a coolant leak or one of these are the problem thermastat,fan clutch,water pump,radiator, try this start car when it reachs normal temp open hood and pinch the top radiator hose if it is hot water is flowing thermastat is good now open radiator with a towel slowly while it is runing see if the water is moving if it is you water pump and radiator are good, now at some point in this time you should have heard the fan get loud if not your fan clutch is not work it mounts to the front of your water pump and is normal to go out because they lose about 250rpm a year which does not give them much of a life, dont forget to check your heater hoses they could be leaking