ATAG · iC, iS and iR range
303PCB fault
This code applies to iC Economiser, iC Combination, iS System, iR Regular. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- Power-cycle or reset once only if the user manual specifically permits it.
- If the message remains, arrange an engineer.
How an engineer may fix it
Check configuration, coding, communication buses, plugs, wiring, power supply and control boards; configure, repair or replace the failed control component.
Manufacturer source: ATAG — Boiler Troubleshooting (2019 & newer i-range). Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.