Louise, since there is several different types and models of Grundfos pumps it would be hard to answer your question. To possible point you in the right direction below are a few links to where you can find some PDF documentation about a popular Grundfos pump the MQ series and direct link to the manufactures "Service Support".
Service support
Grundfos MQ Series Pumps
Grundfos MQ3 45 115V 1 HP Pressure Booster Pump...
Grundfos MQ3 45 115V 1 HP Pressure...
SOURCE: grundfos mq3-45 b
There are several reasons for the pump to go into alarm mode:
1. Overheating
2. Lack of water
3. leak on the suction side causing cavitation
4. Bad circuit board
This pump will try to re-start for 24 hours and will go into Standby mode if there is no water.
What happens if you push Start?
SOURCE: My Grundfos MQ pump intermittantly continues to run
I have the same problem and have found that the inlet check valve has a broken shaft in the center of device. It is a multi-piece plastic unit and it looks as if it is not quite seating and therefore the internal pressure leaks slowly back into my low pressure suction line. The pump will start and the pressure will build then shut off for about 30 minutes then cycle again. I am going to replace the check valve.
SOURCE: I have a grundfos mq3-35b, you turn on the tap
The control unit is faulty, I had that problem twice.. I have now bypassed the control unit and use a normal pump pressure switch to control..works fine..The Grundfos company at first said I had to have air in the lines then admitted the control assy. was at fault. after two new units in three years I did away with the control unit.
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