BoilerFaultCodesIdeal HW light on PCB 40 flashing Fault Code
Also written as HWLIGHTONPCB40FLASHING — same fault.
The Ideal HW light on PCB 40 flashing fault code means: your boiler has detected a fault with the temperature sensor that controls your hot water, meaning it cannot accurately measure how hot the water is.. It appears on 1 Ideal model in our database.
Technical description: Check HW sensor/lead. Sensor may have failed 'low resistance'.
What causes the Ideal HW light on PCB 40 flashing fault?
This fault occurs when the boiler's internal computer receives an incorrect electrical signal from the hot water temperature sensor. It usually means the sensor has developed a short circuit or the wiring connecting it to the main control board has become damaged or loose. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Ideal models that show the HW light on PCB 40 flashing fault code
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Source
Ideal HW light on PCB 40 flashing fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026