Worcester Bosch · Greenstar Si Compact ErP
C4 273Airflow present during previous 24 hours
This code applies to Greenstar 25Si Compact ErP, Greenstar 30Si Compact ErP. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- Do not remove the boiler case or interfere with the flue.
- If the manual identifies a frozen external condensate pipe as a possible cause, follow only the manufacturer's external thawing instructions.
- Otherwise switch the boiler off and arrange a Gas Safe registered engineer.
How an engineer may fix it
Inspect the flue and air path, condensate system, burner, fan or Venturi, sensors and combustion readings; clear, repair, adjust or replace as the appliance instructions require.
Manufacturer source: Worcester Bosch — Greenstar Si Compact ErP Installation, Commissioning and Servicing Instructions (6 720 813 278). Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.