Boiler indicates return temperature sensor fault; may cause poor modulation or lockout.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Defective return NTC sensor, Loose or corroded connector, Damaged return NTC cable, PCB sensor input failure

Safety: Turn off power and let the system cool.

Steps:

Locate return NTC: Find the sensor on the return pipe entering the boiler.
Check fixing: Ensure the sensor is firmly clipped or screwed to the pipe and insulated as designed.
Inspect wiring: Follow the cable to the PCB, checking for damage. Reseat connectors.
Test sensor: Measure resistance at room temperature and compare to spec. Replace if out of range.

Professional help: If a new sensor does not resolve the issue, the PCB may be at fault and should be tested by a Bosch engineer.

Medium 3/5

Return NTC Temperature Sensor

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