The first digit is the circuit: 1xx heating, 2xx DHW / solar, 3xx internal PCB, 4xx external PCB / room sensors, 5xx ignition, 6xx air / flue, 7xx multi-zone. A P in the code (1P1, 5P3) is a warning, not a lockout. Some numbers only apply to a named range — check the model column before you order parts.
Gas work stays with a Gas Safe registered engineer. This table is the display meaning, not a substitute for the Ariston handbook.
| Code | Section | Possible cause | Boiler model |
|---|---|---|---|
| 101 | Central heating | Overheat | All boilers |
| 102 | Central heating | Pressure sensor error | E-Combi Evo, E-System Evo, Clas HE Evo (combi and system) |
| 103 | Central heating | Insufficient circulation | All boilers |
| 104 | Central heating | Insufficient circulation | All boilers |
| 105 | Central heating | Insufficient circulation | All boilers |
| 106 | Central heating | Insufficient circulation | All boilers |
| 107 | Central heating | Insufficient circulation | All boilers |
| 108 | Central heating | Insufficient water (request filling) | All combi boilers |
| 109 | Central heating | System pressure greater than 3 bar | Clas Net One / Clas One Combi, Clas System One |
| 110 | Central heating | Central heating flow temperature probe open / short circuit | All boilers |
| 112 | Central heating | Central heating return temperature probe open / short circuit | All boilers |
| 114 | Central heating | External sensor circuit open / short circuit | All boilers |
| 116 | Central heating | Floor thermostat contact open | All combi and system boilers |
| 118 | Central heating | Heating delivery probe problem | All boilers |
| 1P1 | Central heating | Insufficient circulation indication | All boilers |
| 1P2 | Central heating | Insufficient circulation indication | All boilers |
| 1P3 | Central heating | Insufficient circulation indication | All boilers |
| 1P4 | Central heating | Insufficient circulation indication | Clas Net One Combi, Clas One Combi, Clas System One |
| 202 | DHW | Bottom storage temperature probe open / short circuit | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 204 | DHW | Solar collector temperature probe open / short circuit | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 205 | DHW | Domestic hot water in probe open circuit | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 207 | DHW | Solar collector overheating | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 208 | DHW | Collector frost protection temperature | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 301 | Internal PCB | EEPROM error | All boilers |
| 302 | Internal PCB | Communication error | All boilers |
| 303 | Internal PCB | Main PCB error | All boilers |
| 304 | Internal PCB | Too many resets (more than 5 in 15 minutes) | All boilers |
| 305 | Internal PCB | Main PCB error | All boilers |
| 306 | Internal PCB | Main PCB error | All boilers |
| 307 | Internal PCB | Main PCB error | All boilers |
| 3P9 | Internal PCB | Schedule service | E-Combi One, E-System One, Clas Net One Combi, Clas One Combi, Clas System One |
| 407 | External PCB | Room sensor circuit open / short circuit | Clas HE Evo, Clas HE Evo System, E-Combi Evo, E-System Evo |
| 411 | External PCB | Room sensor circuit open or short circuit zone 1 | Clas Net One, Clas One, E-Combi, E-System |
| 412 | External PCB | Room sensor circuit open or short circuit zone 2 | Clas Net One, Clas One, E-Combi, E-System |
| 414 | External PCB | Room sensor circuit open or short circuit zone 3 | Clas Net One, Clas One, E-Combi, E-System |
| 501 | Ignition and detection | No flame detected | All boilers |
| 502 | Ignition and detection | Flame detected with gas valve closed | All boilers |
| 504 | Ignition and detection | Flame lift | All boilers |
| 5P1 | Ignition and detection | 1st ignition failed | All boilers |
| 5P2 | Ignition and detection | 2nd ignition failed | All boilers |
| 5P3 | Ignition and detection | Flame cut-off | All boilers |
| 604 | Air inlet / flue | Insufficient fan speed | Clas HE R, E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 610 | Air inlet / flue | Thermofuse open | All boilers |
| 612 | Air inlet / flue | Fan speed has incorrect measurement | Clas HE Evo Combi, Clas HE Evo System, E-Combi Evo, E-System Evo, Clas Net One Combi, Clas One Combi, Clas System One |
| 701 | Multi-zone | Zone 2 outgoing sensor defective | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 702 | Multi-zone | Zone 2 return sensor defective | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 703 | Multi-zone | Zone 3 outgoing sensor defective | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 704 | Multi-zone | Zone 3 return sensor defective | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 705 | Multi-zone | Hydraulic separation sensor defective | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 706 | Multi-zone | Zone 2 overheating | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 707 | Multi-zone | Zone 3 overheating | E-Combi Evo, E-System Evo, Clas HE Evo Combi, Clas HE Evo System |
| 701 | Multi-zone | Send sensor defective (zone 1) | Clas Net One, Clas One, E-Combi, E-System |
| 702 | Multi-zone | Send sensor defective (zone 2) | Clas Net One, Clas One, E-Combi, E-System |
| 703 | Multi-zone | Send sensor defective (zone 3) | Clas Net One, Clas One, E-Combi, E-System |
| 711 | Multi-zone | Return sensor defective (zone 1) | Clas Net One, Clas One, E-Combi, E-System |
| 712 | Multi-zone | Return sensor defective (zone 2) | Clas Net One, Clas One, E-Combi, E-System |
| 713 | Multi-zone | Return sensor defective (zone 3) | Clas Net One, Clas One, E-Combi, E-System |
| 722 | Multi-zone | Overheating in zone 2 | Clas Net One, Clas One, E-Combi, E-System |
| 723 | Multi-zone | Overheating in zone 3 | Clas Net One, Clas One, E-Combi, E-System |
| 750 | Multi-zone | All heating zones locked | Clas Net One, Clas One, E-Combi, E-System |
Codes 701–703 mean different sensors on Evo / Clas HE Evo versus Clas One / Net One. Use the model column, not the number alone.
Related: Vaillant fault codes · boiler pressure (108 / 109).