Hi, I have nimbus hj 1000 submersible pump fitted in my cooler , and it stops again & again after working for a small duration & sometimes it never works , please help me what should I do to troubleshoot this problem.

These small pumps are made in Mainland China and usually sell for about $30 so most do not get fixed, rather replaced. The intake is screwed to to motor and can usually be unscrewed and opened. You may find debris inside clogging the small rubber paddle fins, remove it and test the pump with the intake still off. If it does not spin immediately give it a spin with your finger tips. Still not spin, go to Home Depot or pond supply store, not worth further time

Thanks a lot ballengerb 1 , I have concentrated on your answer and tried to open that pump, but all that I was able to do was I have put out its fan which is attached to a circular magnet, and I have not found any screws on that pump, please let me know how to open that pump.

The fan attached to a magnet is the impellor. If its clear then debris is not the issue. Could you spin the "fan" with power still applied?

Yes... but it was still not working.

OK back to my first comment, these are cheaply made and sell for about $30, time to buy a replacement, no further serviceable options available

OK, thanks bally, but can u still tell me how to open that pump

Sorry but without being able to see it I am not sure, it could be glued