BoilerFaultCodesGrant SUP Fault Code
The Grant SUP fault code means: your boiler's temperature sensor has stopped sending the correct signals, meaning the system doesn't know how hot the water is and has shut down as a precaution.. It appears on 2 Grant models in our database.
Technical description: Supply sensor short / Supply sensor open
What causes the Grant SUP fault?
This fault occurs when the main sensor responsible for monitoring the temperature of the water leaving the boiler fails or loses its connection. The most common reason is that the internal thermistor has worn out through constant heating cycles, though it can also be caused by moisture corroding the sensor's electrical pins or a loose wire in the internal harness.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Grant models that show the SUP fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Grant SUP fault code information on this page is sourced and cross-checked against the official Grant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026