BoilerFaultCodesSime AL 81 Fault Code
Also written as AL81 — same fault.
The Sime AL 81 fault code means: your boiler has detected an issue with the air or gas mixture during ignition, often caused by a blocked chimney or an internal sensor problem, and has shut down for safety.. It appears on 13 Sime models in our database.
Technical description: Lockout due to combustion during start-up (blockage) — check flue for blockage; inspect air diaphragm (BF models); verify gas calibration
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Sime models that show the AL 81 fault code
Click a model for the full fault code list and model-specific repair notes.
- Brava One HE Repair: £120-£250
- Brava Slim HE Regular Repair: £120-280
- Brava Slim HE System Repair: £100-280
- Dewy 30/130 HE FS Repair: £120-350
- Dewy 30/50 HE WM Repair: £120-350
- Halstead Trio Repair: £120-280
- Mia 30 Repair: £120-£350
- Murelle One HE Repair: £120-£350
- Murelle Pro HE 25 HO Repair: £100-250
- Murelle Pro HE Combi Repair: £100-280
- Murelle Pro HE MkII Combi Repair: £120-280
- Murelle Pro HE R MkII System Repair: £100-280
- Murelle Pro HE System R Repair: £120-£350
Source
Sime AL 81 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.
- Sime Brava One HE manual (PDF)
- Sime Brava Slim HE System manual (PDF)
- Sime Dewy 30/130 HE FS manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026