The gage is pointing right at the bottom or right at the top and the heater isnt working it just puts out cold air , we just repaced the radiater the fan is working , but the fuel lite is coming on even though there is a full tank. also we know the catilitic converter needs to be replaced , that wouldnt have anything to do with the problem

For the heating blowing cold
Air Lock
Low Coolant Level
Blend Door Actuators not working
Water valve not opening, some are electrically operated (fuse?).

For overheating
Low Coolant level
Air lock
New thermostat faulty
Radiator blocked
Cooling fan not running

When you put in a new thermostat, and refill the system, you have to burp the air out of it. There are videos online, but you fill the system by the radiator (are opposed to the filler bottle) and turn the heater on full to fully open the valve. Then you run the engine and keep topping it up, occasionally squeezing the bottom radiator hose. When the cooling fan kicks in, you know it is up to full running temperature. Rev the engine a few times, and turn off. Then top up one last time and put the radiator filler cap back on. Then top up the coolant reservoir to the line and put the top back on that. All done!

It sounds like your thermostat is stuck closed. Your thermostat also acts as a heater valve to let hot coolant go to your heater core to blow hot air if your thermstat is stuck close it prevents your coolant to run through your radiator to cool down and to blow hot air from your heater core. A way to check to see if your thermostat is working while its at operating temp feel the bottom and top radiator hose if your lower hose is cold and top is hot your thermo is stuck closed but be careful hoses might be very hot

it appears that you have an air lock in the coolant system. bleed out the air by loosing of the highest hose until only water comes out or have the system pressure bled at a radiator shop.

Your radiator cooling fan will only come on when the engine temp has reached the point where it needs to have air forced through the radiator and thus bring engine heat back down to safe operating range. If your heater is not working check to make sure the fresh air vents aren't stuck open (are the controls stiff or not going all the way across?) check also that the heater shutoff valve lever which is usually situated on the firewall at the rear of the engine and can be found easily by tracing the hoses back from the thermostat housing aren't bent and/or seized up.

low coolant sensor is probably just dirty. cold air could also indicate n air bubble in the line. could also be thermostat. try that first.

Sounds like your cooling system has an air pocket in it, and is not completely full. Try this. When the engine is cold. Open up the cap and fill it as high as it will go. Then start engine, Check level again and top off again with the heater on full heat put cap back on. Watch your temp gage. When temp get to around 215-220 the fan should tart up. Have someone keep idle slightly elevated at fast idle. Your temp should syart to lower. As soon as fan shuts off, and still hold a fast idle slowly open up the cap again, and top off the system again. This should have the system completely full, and prged of air. The make sire your reserve tank is at the proper level. See if this takes care of your problem. Your heater is the highest thing in your cooling system. If there is air in there, you will not get any heat. That would explain why you only get heat while driving..

did you buy the heater cores new heater core and radiator is two different things sounds like you thermostat is bad touch your bottom hose and your top are they both hot if ones hot and ones cold water is not circulating bad thermostat is the heater core put in right

Concerning the heater problem.Sounds like you have an air pocket in the system.To remove the air pocket take off the radiator cap in the morning when the car is cold,fill radiator to top.Start the car.Turn on heater to high.Allow car to run until it gets to operating temperature.Now you will see the fluid in the radiator start to go down.This means the thermostat is opening.As fluid goes down fill radiator to top (with 50/50 antifreeze/water mix).You may have to wait a moment for it to continue to move down.Once it stops and the radiator fluid isnt going down,slowly bring up the rpm's (by either turning the throttle with your hand or have someone put their foot on the gas) to about 2000rpms and hold the rpm's there steady.As you hold the fluid will go down a little more.As it does fill to the top again and quickly put the radiator cap back on while the rpm's are steady at 2000.After the cap is correctly on release throttle.This should remove the air posket.

sounds like u hav an air bubble in radiator from cold take radiator top off then rev engine to see if it bubbles if it does do this till it stops bubbling then top up wen it cools down then put radiator cap on then check heater gets hot wen warm then this is solved

if you replaced the thermostat, you have an air bubble trapped in the engine. So, you don't have any coolant running through the heater core, therefore no heat.
sometimes hard to "burp" air bubble. Try this. Park park facing uphill and jack up front of car too. You want to get radiator opening higher than top of engine. Then remvoe radiator cap (cold engine), fill radiator, then fill coolant reservoir to full point. Install radiator cap but leave it loose so air can escape. Start engine until upper radiator hose going into radiator is hot. This should burp air. Repeat a few times while adding coolant mix (50/50 antifreeze) to keep radiator full. Be careful not to remove radiator cap when engine is hot.
lwt us know how ou make out. There is a more complicated procedure but you need tools and knowledge of where air bleed is on top of engine