Baxi · Combi & System 2 platform
E.00.21Flue-gas temperature sensor short-circuited or reading above range
This code applies to Baxi 200 Combi 2, Baxi 400 Combi 2, Baxi 600 Combi 2, Baxi 800 Combi 2, Baxi Assure 500 Combi 2, Baxi 600 System 2, Baxi 800 System 2, Baxi Assure 500 System 2. 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: Baxi Professional — Boiler Error Codes. Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.