| E.00 .04 | Return temperature sensor not connected. | A loose/oxidised connector or an open wire in the harness 7773502 causes the return NTC to be 'unseen' by the PCB. Check pin-to-pin continuity of the harness between the sensor and the PCB and the ohmic resistance of the NTC. |
| E.00 .05 | Return temperature sensor in short circuit. | A chafing or crossing between conductors of the harness 7773502 can close the NTC circuit, and the PCB interprets it as a short circuit. Check the NTC and harness insulation; if the NTC measures correctly, inspect/replace the harness. |
| E.04 .02 | Flow temperature sensor not connected. | Loss of contact in the NTC flow connector within the harness 7773502 causes 'sensor not connected'. Check continuity of the harness to the PCB and the ohmic reading of the NTC. |
| E.04 .05 | Flue gas sensor not connected. | If the connector/branch of the flue gas sensor in the harness 7773502 is loose or open, the PCB detects 'flue sensor not connected'. Check continuity of the branch and connection at PCB/sensor. |
| E.02 .17 | Permanent lack of communication between the gas valve and the boiler PCB. | A faulty contact or cut wire in the harness 7773502 (gas valve branch) prevents communication/commands from the PCB. Check continuity of the branch, connector firmness, and control voltage according to the model's diagram. |
| H.01 .00 | Temporary communication failure between the gas valve and the boiler PCB. | Vibration or moisture can cause intermittencies in the valve branch within the harness 7773502; the equipment reports temporary communication loss. Check connectors, fastening, and continuity; if it reappears, replace the harness. |
