BF50D 4 stroke Honda fuel injected outboard

[p4j]

I have a 2008 BF50D 4 stroke Honda fuel injected outboard. The alarm engine light comes on and buzzer sounds about every second. This happens after running for about 20 to 30 min and when I reduce to low speed. If I turn engine off and restart the fault clears but will again come on after taking off again. Can this be the O2 Sensor causing this problem. The engone does not faulter when the alarm comes on.