890 Freeze condition detected

What the control is telling you

At operation, the heat exchanger temperature reading is too low, indicating a possible freeze.

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

Likely causes

• Frozen boiler lines, heat exchanger or CH system
• Loose, broken or damaged thermistor wiring
• Scale build-up on thermistor
• Faulty thermistor
• Failed PC Board

Test and repair steps

1. Check that the boiler lines, heat exchanger and central heating system are not frozen.
2. Inspect the thermistor harness back to the PC Board for loose, broken or damaged connections.
3. With the boiler supply lines drained, remove the thermistor and clean off any scale.
4. With water isolated, measure thermistor resistance on a 50K-ohm range at the thermistor side of the connector; warming the bulb (e.g., in your fingers) should lower resistance, and cooling it should raise it.
5. Check at PC Board connector CN7 pins 7 and 10 and compare against Appendix A in the source manual (p.51); replace if off by more than 5°F.
6. If readings are good, reinstall the thermistor with its small O-ring intact (lightly greased), turn water back on and check for leaks.
7. Fire the unit; if 890 returns, check the whole system for frozen pipes. If nothing is frozen, replace the thermistor.
8. If the code returns after a new thermistor, replace the PC Board.