380 Exhaust thermistor fault

What the control is telling you

Fault detected in the exhaust temperature thermistor in the exhaust duct housing.

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
• Thermistor unplugged (reads -40)
• Failed exhaust or return thermistor
• Wrong gas type, orifice or A0 setting
• Poor clearances at unit/vent termination
• Clogged heat exchanger fins

Test and repair steps

1. Inspect the thermistor harness from the sensor back to the PC Board for loose, broken or damaged connections.
2. Open Performance Data 06 and compare the exhaust temperature with a thermometer at the same spot. Within 5° = OK; more than 5° off = continue; -40 = unplugged or poor connection.
3. Remove the thermistor from the exhaust duct housing.
4. Measure it on a 20K ohm range; resistance should drop when warmed by hand and rise against ice. 150K and smaller: CN7 pins 3 and 6. 175K and higher: CN7 pins 3 and 14.
5. Compare against Appendix B Exhaust Temperatures in the source manual; replace if more than 5°F off.
6. Also check the return thermistor resistance (same values as the exhaust thermistor).
7. Confirm the gas supply matches the rating plate and the gas valve has the right orifice (black = NG, grey = LPG, white = NG high altitude above 2000 ft).
8. Confirm Parameter A0 matches the supplied gas (A = Natural Gas, b = Propane).
9. Confirm proper clearances around the unit and vent termination.
10. If heat exchanger fins are clogged, clean with 120 psi compressed air or replace the heat exchanger.