Q: My heater is blowing cold air when idling - 2005 Nissan Altima

Hello. A cold blowing heater can absolutely be caused by low coolant. You are on the right track. However, if that coolant is not being circulated through the heater core, the warm coolant will not cycle into the core, and the air blowing through will not warm up. You may have an issue with the thermostat or the heater core.

You may want to think about why the coolant was low. If it was abnormally low, there may be a leak, and this will need to be fixed first. If the lack of coolant was from normal use, you will want to look into the thermostat. The best way to test this is to test the operation of the cooling system. This can be done in several ways, and you will want to look further into the process separately if that is the case. You will also want to look into the heater core. They can become clogged and prevent the flow of coolant. Be sure to check, at this point, for a leak in the system.

I recommend having a certified mechanic, such as one from YourMechanic, come to your location to determine why the heater is blowing cold air and check for any possible leaks in the cooling system.