2005 nissan altima heater blows cold air when stopped and blows hot air when going down the highway. What could be the problem?

Change back to the correct t-stat, the main air door thermistor that goes from hot to cold is stuck or going bad or some cases the dash temp control is going out.

Heater core may be clogged. Try flushing it to see if water flows through freely. Also check the heater water control valve for restrictions.

I have the same problem. When my dealer changed my coolant I now seem to have a stuck thermostat. They are denying they did it. I haven't changed the thermostat yet but that is the problem.

heater core cloge

you have a air bubble in the system. I would suspect there is a leak somewhere in the system. You should have it looked at and have the system pressure tested to insure a leak is not present.

you have air trapped in cooling system. Go to a shop that uses evacuate & fill equipment and have them flush system and burp air. SHould cost about $65. Make sure they have evac/fill equipment

you may have a sticking thermostat make sure you have enough anti-freeze in it and it does not have a air bubble in it give these websites a try www.alldatadiy.com and www.autozone.com if all fails stop by your local library and get your hands on a Haynes auto repair manual for your car wish you the best of luck Michigan Man.

Check the coolant level.If you are low it will stop flowing thru the heater core at low RPMs.

Hi!
It appears we have an Air lock scenario and you will need to perform a system Bleed.
Park the vehicle on level ground, when cold remove coolant filler cap, start engine and leave to idle, turn heater on full and blower to max. When engine reaches operating temperature watch and listen near coolant filler, keep clear as gurgling and hopefully a boil over should occur. Top up with very warm coolant and wait as it may do it again.
Check for heat inside vehicle if warm replace coolant cap but keep an eye on temperature gauge as the ~Air lock may have moved on from heater matrix/core so proceedure needs to be carried out again from COLD.

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It sounds like the fix is a simple one. Just change the thermostat. If you don't know how to do this go to an auto zone or similar auto parts store and they will assist you. good luck and please rate my answer.