Heater not producing enough heat. How do I determine the difference between poor exchange from the heater core and a faulty thermostat without throwing my money in the wrong direction. At idle the engine temp. reaches about half way on the temp guage but takes about 15min. to get there with the cooling fan coming on at this point. The temp. on the guage drops very quickly after driving with the ambient temp. at about -3 deg. celcius. Guage sits at the solid mark on the cold end of the scale. Is it possible to do a descent job at reverse flushing myself or is it necessary pay the big bucks at the shop.

If the heater core hoses are hot but there is no heat coming out of the vents, and the vehicle is also overheating, there are several possible issues that could be causing this problem. Here are a few things to check:

1. Coolant level: Check the coolant level in the radiator and reservoir. If the level is low, it could cause both the overheating and lack of heat. Top up the coolant to the correct level and check for leaks.
2. Thermostat: If the thermostat is stuck closed, it could be preventing coolant from circulating through the engine and into the heater core. This could cause both the overheating and lack of heat. Consider replacing the thermostat if it is found to be faulty.
3. Water pump: The water pump circulates coolant through the engine and heater core. If the water pump is not working properly, it could be causing both the overheating and lack of heat. A mechanic can test the water pump to determine if it is functioning correctly.
4. Heater core blockage: If the heater core is clogged or blocked with debris, it can prevent hot air from circulating through the cabin. This could cause a lack of heat. Flushing the heater core may help remove any blockages.
5. Cooling system blockage: A blockage in the cooling system, such as a clogged radiator or hose, can prevent coolant from flowing properly and cause overheating. Have a mechanic inspect the cooling system to determine if a blockage is present.

These are just a few possible causes of the overheating and lack of heat. It is recommended to have a qualified mechanic inspect the vehicle to diagnose and repair the issue.

as of reading forms from other mazda sites. its a blocked core. they seem to have a problem with them, and needs to be backflushed. others have said the controller for the heater flow is not opening up fully, others have changed the settings to recirculate the air so its using the inside air and not the outside air and this has worked. but its been a problem from some 2009 models and never been rectified.

Not really, the heater core would still get hot enough to produce enough heat and usually the hoses will fall to the right spot making it hard to put them on the wrong ends. Why was the heater core changed? was it clogged? Did you flush the engine out to prevent the new core from clogging up? Have you looked into it being a stuck open thermostat?

there are several likely reasons -

Poor/no flow of heated coolant through heater core (heat exchanger - mini radiator in cabin) this could be due to closed valve or blockage in pipe of core.

No air flow - bad blower, blend door problem

Hopefully it won't come down to a core replacement!

It's common to see the mini heater cores clog up a bit and deliver poor heat. Most likely you need a new heater core. Not really a DIY job unless you're experienced. You could try pulling the heater hoses and flushing it out with the hose.

If both heater hoses are getting hot then you know the coolant has to be flowing and the core should be hot.
What you will have to do is locate the blend air door servo.
The blend air door is a door (or flap) between the heater core and the A/C evaporator (even without a/c there is still a door to control the cabin temp)
Early vehicles with the slider controls used a cable but the later vehicles with a rotary knob or digital buttons use a servo to rotate the door mechanism.
The servos can go bad and the switches as well.
But from the sounds of it, your door is not moving to allow the heat through the vents.

check the heater valve or the flap inside your ductwork

If the car is not providing heat and thermostat is good, then your problem is a faulty heater core. Sometimes when heat cores wear out, they sometimes give off an anti-freeze smell.
You can go to autozone.com and register for full free online repair manual with everything you could need for your car.

The heater in your car depends upon a flow of hot coolant going through the heater core in order to get hot air into the car. If the heater core is partially plugged, it will restrict the flow. If you have a head gasket that is putting exhaust into the cooling system, it will eventually get into the heater core. When this happens there is no coolant in the heater core to make the air warm. If the water pump vanes have corroded away the result will be very poor coolant circulation causing the heater to not blow hot air. If your car is equipped with a heater control valve that is not functioning properly, the coolant flow will be restricted. One or a combination of these factors is likely the culprit in your case.

if the car is not overheating, the thermostat is working fine.... replace or flush the heater core multiple times to try and eliminate sediment build-up.

remove the heater core lines the two hoses that go to the heater core, take a splice tube either metal our plastic your part store will have either one. take the splice tube and install it in one of the lines add some hose to that tube make the hose long enough to run out side of the engine compartment and on to the ground. take the other hose and install a flushing tee, hook up to a garden hose and trun the water on . what you are trying to do is flush the heater core , you are also looking for blockage off the heater core, if you little to no flow i would suggest replaceing the heater core