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Heater working intermittently

Question for anyone in the know. I have a 2005 Nissan Sentra with 65k miles. After having the “routine” 60k tune up, my heat stopped working. Not great timing as it is winter. Searching this board and several others, I narrowed it down to a thermostat or a clogged heater core. I have replaced the thermostat, and flushed the heater core. Now, the heater works occasionally. This is way better than it not working but was looking for some advice on how to make it work all the time.
I flushed the core for about 5 minutes. I have heard that you should flush it for longer, but everything was coming out clear and easy after 5 minutes so I assumed it had done the trick.
I’d appreciate any suggestion/input.
Thanks in advance

As long as your fan is pushing air through the ducts as it should be then I would look at two other areas. First make sure all the air is out of the cooling system. If the air is out and your coolant is full…you may be looking at a bad water pump. I’ve only seen a few bad ones but the vanes inside the water pump can break off or simply corrode away. You might need to see a good independent mechanic to get your heating system squared away.

Did you flush the heater core in both directions?
Did you get a strong water flow out of the heater core?
Some guy on this website said he put CLR (calcium and lime remover, I believe) in his heater core overnight and flushed it out the next morning. He said it worked perfectly after that.

Do not use CLR or Lime Away in a heater core. If you look on the bottles it states “NOT TO BE USED ON ALUMINUM.” Guess what heater cores are made of?

There’s probably air trapped in the cooling system. To remove the air, get the engine up to operating temperature. With the engine idling loosen the upper radiator hose clamp. Take a small flat bladded screwdriver and slip the tip in between the upper hose and the radiator hose neck. Allow the engine to continue to idle until all the air is purged from the cooling system and nothing but coolant come out. Remove the screwdriver and retighten the hose clamp.

Tester

@Tester I wasn’t the one who used it.
A guy we were helping on this website decided to use it and was happy with the results.

Engine temp as measured by the dashboard guage same as before? Then this sounds more like coolant isn’t filled to spec, or air remains trapped in the cooling system. Some cars are considerably more difficult to air bleed than others. I’ve always used a varient of what Tester suggests above, but also Google “How to bleed the air from the cooling system of a Nissan Sentra”. There may be others who have this problem and figured out how to solve it.