BoilerFaultCodesSime AL 18 Fault Code
Also written as AL18 — same fault.
The Sime AL 18 fault code means: your boiler has detected that waste water cannot drain away properly, usually because the discharge pipe is blocked or the internal trap is full.. It appears on 15 Sime models in our database.
Technical description: Condensate level fault — check condensate drain pipe for blockage; check siphon not clogged
DIY fix possible
Locate the plastic pipe leading from the boiler to the outside drain.Check if the end of the pipe is submerged in water or blocked by debris/ice.If the pipe is frozen, gently pour warm (not boiling) water over it to melt the ice.Restart the boiler using the reset button.
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Sime models that show the AL 18 fault code
Click a model for the full fault code list and model-specific repair notes.
- Brava One HE Repair: £80-140
- Brava Slim HE Regular Repair: £80-160
- Brava Slim HE System Repair: £80-140
- Dewy 30/130 HE FS Repair: £80-160
- Dewy 30/50 HE WM Repair: £80-160
- Format DGT HE Combi Repair: £80-160
- Format DGT HE System Repair: £80-160
- Halstead Trio Repair: £80-160
- Mia 30 Repair: £80-160
- Murelle One HE Repair: £80-160
- Murelle Pro HE 25 HO Repair: £80-160
- Murelle Pro HE Combi Repair: £80-160
- Murelle Pro HE MkII Combi Repair: £80-160
- Murelle Pro HE R MkII System Repair: £80-140
- Murelle Pro HE System R Repair: £85-160
Source
Sime AL 18 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