| E33 | The air pressure switch is closed before the ignition sequence. | The pressure switch remains closed at rest (membrane stuck or wiring/connections reversed). Check with a multimeter that it is open at rest and inspect/clean the connections and pressure tubes. |
| E34 | The air pressure switch does not close when the fan operates. | The pressure switch does not change state when the fan starts (contacts open due to internal fault or insufficient ΔP). Check continuity during start-up and measure depression with a differential manometer; inspect pressure tubes/connections. |
| 607 | Authorization of the flue pressure switch preceding the ignition sequence. | The pressure switch appears 'ON' when it shouldn't (contacts stuck or connections improperly made). Verify that it is open at rest and that the connections do not create undue depression. |
| 608 | No authorization from the flue gas pressure switch with the fan on. | The pressure switch does not close despite the fan turning (defective diaphragm/switch or insufficient ΔP). Check continuity when starting and measure ΔP; inspect tubes and chimney obstructions. |
| 6P1 | Delay in air pressure switch authorization. | The pressure switch takes time to close due to being sluggish or due to limit depression. Check the closing time with a multimeter and measure ΔP during pre-ventilation; clean/adjust connections. |
| 6P2 | Air pressure switch opening during operation. | The pressure switch opens during operation (unstable contact or draft/pressure drop). Check continuity in operation and measure sustained ΔP; inspect tubes, obstructions, and the pressure switch itself. |
