When I warm up the car,it heats fine.But when I start driving,the temp gauge goes down to the C,sometimes lower,and don't get heat.When I'm at a stop,the temp warms up again.I just changed the fluid like a week ago.Any idea as to what might be wrong?

 

Bad Radiator thermostat. Probably stuck open.

 

to what d280c said - you need a new thermostat - its a 15 dollar part and easy to replace.

 

I'd agree. This is a $15 part....I think you have to get them at the dealer. I ordered mine from thepartsbin.com and it came in a nissan box. No special tools required that I can recall. You'll need to drain out the coolant into a bucket or something clean if you want to reuse it...since its so new there is no reason you wouldn't want to.

Most thermostats will fail open to prevent the engine from overheating. Until you get it fixed there may be some negative impact on your fuel economy, aside from the heater not working.