Today I arrived on site to address the heating system issue. Issue: Heating system not working for the Heating and DHW zone. Diagnosis: Found the high limit switch open. Resolution: Reset the unit. Verification: Ran and tested the system; now back up and running. Upon arrival at the property, contact the customer to have access to the boiler room. The client explained to me that the system is not heating the house. I proceeded to verify the unit. I found the unit in lockout mode. Error code #A03. I proceeded to verify the electronic components of the unit. Reset the unit. I ran and tested the system for about 40 minutes. The unit showed no faults on this occasion. Disassemble and clean the HLS of the unit. I contacted the client again to explain the situation with the system. I suggested and recommended to the client that if the unit returns and fails again. It is recommended to replace the control module of the unit and the High Limit Sensor. I explained to the customer