ALL 03 Medium

Sime ALL 03 Fault Code

Also written as ALL03 — same fault.

The Sime ALL 03 fault code means: your boiler has detected that the water pressure inside the heating system has risen too high, which may cause it to stop working to protect the internal components.. It appears on 7 Sime models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: High water pressure in system
Typical repair cost £80-120 (including labour)

DIY fix possible

Locate a radiator in your home, preferably one upstairs.Place a cloth or small container under the radiator bleed valve.Open the bleed valve slightly using a radiator key until water starts to trickle out.Allow a small amount of water to drain until the pressure gauge on your boiler drops to the green zone (typically 1.0 to 1.5 bar).Close the valve tightly and check that the boiler restarts.

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Sime models that show the ALL 03 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Sime ALL 03 fault code information on this page is sourced and cross-checked against the official Sime service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026