Did i break my av receiver?

I've had a Sony str-dh590 av-receiver for 2 years and now it started giving "protect" message and won't stay on longer than 3 minutes even without any speakers connected. I have a Simex 5.1 set and it has an active subwoofer with impedance of 4 ohms, connected to the receiver with RCA wires. The receiver is rated for 6-16 ohms. Since the subwoofer has its own amp, shouldn't the impedance difference matter?