I completed the recommendations including trying a new cartridge. I installed the new cartridge. The printer reset with no error codes. I tried a test print job. It printed 2 pages , stopped and gave me same error code Er11 again.
So I determined it was a printer problem. Since it was out of warranty, I bought a new HP printer.
Hopefully I don't have more future problems as the defective printer was less than 2 yrs old.