2004 Nissan Maxima heater doesn't work when the car is idling. Only when in motion it does work. What could be the problem?

 

Air bubble in your coolant system. Park on an inclined hill with front of car higher than back about 10% incline should be fine. Let car cool down. Open coolant resevour, turn car on "running". turn heater on full with hottest temp. Wait until car reaches operating temp. watch the coolant during this process to make sure it doesn't overflow onto the ground. If it starts to, shut the engine off. I would also put something under car to catch any that boils over. The reason you need to do this, is to get the air out of the system. The coolant doesn't get all the way through the engine until the thermostat reaches a hot enough temp to open allowing passage. Air will travel up and burp out the fill cap. Once this happens, shut car off and let cool down again, repeat as necessary. Make sure you fill up with coolant as needed. Should take less than 30 minutes.

 

Not quite sure what you mean by heater doesn't work.

Do you mean that the Fan does not work or that you have no heat?

 

Start with the easy stuff. Check the coolant level. If its low, fill it...

 

I would agree if there wasn't a whole different part of the engine that the coolant goes to besides the part where you get the level. You need to be at operating temp before you can accurately measure the coolant level.