Boiler display shows a sensor error; boiler may not fire or may cycle rapidly.

Safety: Isolate power before accessing internal wiring or sensors.

Engineer steps:

• Identify the flow and return NTC sensors on the Grant boiler primary circuit.
• Disconnect the suspect sensor and measure its resistance at a known water temperature; compare with Grant’s resistance/temperature chart.
• Inspect the sensor wiring loom for chafing, loose connectors, or water ingress at plugs.
• Replace any faulty NTC sensor with a genuine Grant-specified part and clear the fault on the control panel.

Users should not attempt to bypass or bridge sensors; this is unsafe and can damage the boiler.