Have had thermostat replaced, still runs cold in the heater

Most likely the thermostat broke. I expect that you have an older Hyundai and never had the anti-freeze changed. When the engine is cold the thermostat is closed which causes the engine to warm up more quickly. A warm engine runs more efficiently (less gas) and more smoothly. When the engine temp reaches a certain temp the thermostat opens and then allows the hot engine coolant to run through the lines to the heater core. The heater blows air over the heater core which produces the warm air that warms up the cabin. Thermostats used to be cheap and easy to change. I would suggest a web search for how to replace the thermostat. Something like 'hyundai accent 2002 replace thermostat".

The dash gauges are notoriously inaccurate. But it is possible that the thermostat is stuck open. Does the upper radiator hose get hot after engine warmed up? Also feel both heater hoses-when engine is warm, both should be hot-indicating the coolant is circulating through the heater core. If heater hoses are hot, and you still have cold air, the temperature blend door on the heater case may not be operating.
Try a new thermostat and see if engine warms up faster and better.

Could be low on coolant, or the thermostat may not be closing, or there may be a problem with the blend door under the dash.
The engine thermostat is closed until the engine warms up, so if it is stuck open the engine never warms up.
The blend doors under the dash direct air thru the heater and the vents.

If the thermostat is not working the engine will not warm up. So the temp gauge should show cold after running the engine for 10 or 15 min.
If the engine is warming up but the heater hoses are not warm, you could have air in the cooling system.

No, this is not your case. If the thermostat is bad any warning light would be lit on the dash.
First time you might check the thermostat operating.
Heed of antifreeze coolant temperature!!!
The thermostat location for 2000 VW Jetta 1.9 TDI, is on back of the alternator at the bottom side of the bracket.
IMPORTANT: It is NOT necessary to replace the alternator if you want to change the thermostat !!!
Thermostat checking:
1. Cold engine. Start up the engine and watch of the water temp indicator. After first value 60 Celsius Degree you still warm up the engine until the lifter (pointer) reach 80 C deg. (second mark after 60 C deg.). At this time you let engine running at idle speed and check with your hand the thermostat plastic housing and hose temperature in bottom left side of the engine. Heed of alternator driving belt and ventilator fan !!!
2. If the thermostat is good the hose must be cold until the water temp indicator reach almost 90 C deg.
3. The thermostat opening temperature is 87 C deg.
4. If the hose is already warm the thermostat is out of order.

Thermostat location, Part number: 044121113, in german - "Kuehlregle" (Price around $25):


Important note: Always replace the thermostat round seal, position 7 in the above picture.

Please do rate my response. Thanks!

This sounds like a bad water thermostat or a cooling fan relay stuck on Run. The thermostat should be under the top housing of the motor where the big radiator hose meets the engine.

The other method of cooling a hot motor is the electric fan attached to one side of the Radiator. This fan may run more when the A/C is on, but when you are using the heat it should not run constantly. This fan should be off when the motor is cold.

In some cases the sensor for the fan will make a trouble Code and light the "check engine" light on the dash. You can get a free scan from Autozone or Oreillys.

Once you get heat in the motor your heater should work quicker. If the motor is hot and the heater is not, feel the smaller hoses running from the motor to the Firewall. They are about 5/8 to 3/4 inch diameter hoses. They should be hot with the heater on. If they are hot, and the heater in the car is only warm or cold, you have a ductwork problem under the dash. If the hoses are not hot, you have a flow problem. With the flow problem, either the heater core is plugged or a water valve is not turning on.

I hope my solution has identified the problem and given you a way to trace it. Start with the thermostat, new gasket, and some antifreeze to replace spillage. If you did have a warning light, either disconnect the NEG battery cable for 10 minutes or ask Autozone or Oreillys to clear your car with their scanner. Old error codes will stay in memory for 25-35 Startups if you don't clear them.

WITH ENGINE FULLY WARM, FELL BOTH HEATER HOSES THEY SHOULD BOTH BE HOT. IF ONE IS HOT AND THE OTHER IS COLD HEATER CORE IS LIKELY PLUGGED. IF BOTH ARE HOT THEN YOU HAVE A BLEND DOOR PROBLEM UNDER THE DASH.

Check vacume lines on heater boxes under dash doors on your heater box are opening and closeing check the vacume lines on engine also sounds like a vacume leek

i had this exact same problem with my 98 explorer. your blend actuator (blend doors) are not morking. these are on the passenger side under the dash. getting to them will be dificult, but it can be done. you will need to take apart the heater box, to fix them. (the big black box under the dash).

2000 jetta warms up when idling, but gets cold as soon as I start driving.