How much does it cost to fix Grant error code OPn?

Typical repair cost for the Grant OPn fault is £120-£220 including labour.

OPn High

Grant OPn Fault Code

Q: What does the Grant OPn fault code mean?

Your boiler has detected that a temperature sensor cable has disconnected or become faulty, preventing the system from monitoring water flow correctly.

Also written as OPN — same fault.

The Grant OPn fault code means: your boiler has detected that a temperature sensor cable has disconnected or become faulty, preventing the system from monitoring water flow correctly.. It appears on 2 Grant models in our database.

Technical description: HP flow sensor open / HP return sensor open

What causes the Grant OPn fault?

This fault occurs when the boiler's control board loses communication with the sensors that monitor the temperature of the water entering or leaving the unit. It is usually caused by a sensor internal failure, a loose connection, or damage to the wiring harness that prevents the electrical signal from reaching the main controller.

Typical repair cost £120-£220 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Grant models that show the OPn fault code

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

Source

Grant OPn 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