My Altima is overheating, when I get home after work the water reservoir is bubbling and the strange thing is that the fans are not on. The temperature is slightly higher than half the scale. I'm not sure if the fans ever turn on so I tried leaving the car in idle and then reving it a couple of times, this during 10 minutes. The water started bubbling again but the fans where off!!! I checked the fuses, and they're ok. I don't know how to check the relays, what else can I check or replace????