BoilerFaultCodesMain E321 Fault Code
The Main E321 fault code means: your boiler has detected a problem with the sensor that monitors the temperature of your hot water, meaning your taps may not get as hot as they should or stay consistent.. It appears on 2 Main models in our database.
Technical description: Hot water negative thermistor coefficient (NTC) error
What causes the Main E321 fault?
This fault occurs when the sensor responsible for measuring the temperature of your hot water fails or sends an incorrect signal to the boiler's control board. The most common reason is that the sensor has physically degraded over time or developed an electrical internal short, preventing it from tracking heat accurately. In some cases, it can also be triggered by loose or corroded wiring connections between the sensor and the main computer.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Main models that show the E321 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main E321 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026