A P3 error means SunPump has detected a water flow issue OUTSIDE of SunPump. SunPump requires 30-seconds of good flow before the compressor will start, so a P3 flow problem means SunPump will not operate until flow is satisfied.
This is not a SunPump problem, it is typically a plumbing or pump problem that should be addressed by the installer. SunPump does not come with a pump, it is an external component that the installer connected, so the malfunction is not inside the SunPump box - even though our controller shows P3 error.
The Normally Open switch requires adequate water flow to close the electrical connection by the pressure of flow against the Paddle in the stream of the pipe. What can go wrong is usually a mechanical problem where the paddle rubs against the pipe wall, or is not broadside against the water stream. The electrical switch is simple and reliable, if it has operated fine it should not be the first thing to test with a multi-meter.
We have mostly seen pump issues, where the pump is backwards, not connected to power, or has an Up-Down-Up inverted U-shape water pipe that has formed an Air lock. These issues are easy to spot if you patiently examine and verify the installation.
If friction is suspected, check the length of the paddle is shorter than the threaded pipe well depth with a Popsicle stick.
It is possible to snip/trim the paddle length if required.
Close both external isolation ball valves before removing the flow switch, and have a towel or pan to catch the water.