4 | P a g e
40 Description: Return sensor error
Cause: The return sensor T2 value out of range.
1. Check full T2 wiring harness and connectors for signs of damage, corrosion, or loose contact.
2. Use a multimeter to check T2 sensor resistances are in line with the return temperatures.
• The return temperature can be identified using the A menu and A1.
• The resistances can be taken at the connector at the PCB X4 pins 3 & 4. This checks the
wiring harness continuity at same time. Disconnect the connector from PCB 1
st
to ensure no
additional resistances are given through the PCB. Resistances are found on page 67.
• If the readings are incorrect, disconnect the wiring harness from the sensor and check the
sensor readings without the harness to prove if the sensor or harness is the fault. Replace
parts as required.
*Note thermistors may need cleaned and not always replaced*.
3. If error 40 remains after the above has been checked, replace PCB.
50 Description: T3 Hot water sensor error.
Cause: The values of T3 boiler sensor out of specification.
Combi & Economiser boilers.
Cause: The values of the T3 hot water sensor outside specifications.
1. Check full T1 wiring harness and connectors for signs of damage, corrosion, or loose contact.
2. Use a multimeter to check if T3 sensor resistances are in line with the return temperatures and per the table
on page 67.
• The DHW temperature can be identified using the A menu and A2.
• The resistances can be taken at the connector at the PCB X7, pins 8 & 9. This checks the
wiring harness continuity at same time. Disconnect the connector from PCB 1
st
to ensure no
additional resistances are given through the PCB.
• If the readings are incorrect, disconnect the wiring harness from the sensor and check the
sensor readings without the harness to prove if the sensor or harness is the fault. Replace
parts as required.
*Note thermistors may need cleaned and not always replaced*.
4. If error 50 remains after the above has been checked, replace PCB.
Additional checks for iS & iR boilers with Hot Water Priority.
1. Check the Hot water priority DHW NTC sensor is connected to the boiler.
2. Check the boiler sensor wires for loose contacts and breaks.
3. If the cable has been extended, check the wires at the junction box/block connector.
4. Check if the sensor is installed correctly.
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