353 CH supply thermistor fault

What the control is telling you

Fault detected in the supply thermistor, which relates to central heating only.

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi and Solo)

Likely causes

• Loose, broken or damaged thermistor wiring/connectors
• Thermistor unplugged (reads -40)
• Scale or deposits on the thermistor
• Failed thermistor (resistance out of range)
• Faulty PC Board

Test and repair steps

1. Inspect the thermistor harness from the sensor back to the PC Board for loose, broken or damaged connections.
2. Caution shut off the water supply and drain the system down before going further.
3. Open Performance Data 03 and compare the displayed temperature with a thermometer at the same spot. Within 5° = sensor OK; more than 5° off = continue; a reading of -40 means unplugged or poor connection.
4. With the lines drained/isolated, pull the thermistor and clean off any scale or deposits.
5. Measure resistance on a 20K ohm range; it should drop when warmed by hand and rise on ice. 150K and smaller: CN7 pins 5 and 6. 175K and higher: CN7 pins 5 and 14.
6. Compare readings against Appendix A Water Temperatures in the source manual (p.21 references it); replace the sensor if it is off by more than 5°F.
7. Cross-check the readings with the controller's performance data; if correct, reinstall the thermistor with its small O-ring intact and lightly lubricated.
8. Restore water/power and check for leaks around the thermistor.
9. If there is no leak and resistance is correct, replace the PC Board.