issue

I have a MaxFlo on my solar panels heating system. For different reasons, the solar system uses a separate plumbing system than the main one, handled by a different pump. The pump is activated automatically by the solar control system, whenever it makes sense based on air/water temperature...

Nearly zero psi. Return flow looks normal. I just replaced pressure gauge and washed the cartridge filters today but didn’t solve the issue. My pump basket is clean. What else should I check? Please