IBC DC Series Gas boiler

Contobr

Recently my boiler has given me a code "L0 P" hot water still works in the house but baseboard heat will cut out. If I power cycle the system it's good for awhile then I get the code again. Pressure gauge on the fill is at 15psi I raised is 1 psi just to see if it fixed it with no change. Pressure per gauge stays constant set at 15psi. What is the first thing I should be checking? What's the proper way to bleed this system of air.