303 Secondary system thermistor fault

What the control is telling you

Fault in the secondary system temperature thermistor used for cascade heating control between boilers.

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

Likely causes

• Parameter 70 not set to 'b' (in use)
• Loose, broken or damaged thermistor wiring/connectors
• Thermistor unplugged (reads -40)
• Damaged or failed thermistor
• Faulty PC Board

Test and repair steps

1. Confirm Parameter 70 (system thermistor control, used for cascade heating between boilers) is set to 'b' (in use).
2. Inspect the thermistor harness from the sensor back to the PC Board for loose, broken or damaged connections.
3. Caution shut off the water supply and drain the system down before going further.
4. Open Performance Data 33 and compare the secondary system temperature with a thermometer at the same spot. Within 5° = OK; more than 5° off = continue; -40 = unplugged or poor connection.
5. Remove the thermistor and inspect for damage.
6. Disconnect and measure at the thermistor side on a 50K ohm range (CN11 pins 10 and 14 on PC Board); resistance should drop when warmed by hand and rise against ice.
7. Compare against the typical resistance values listed in the source manual (p.15).
8. Cross-check with the controller's performance data; if correct, reinstall the thermistor.
9. Fire the unit; if code 303 remains, replace the thermistor.
10. If the code returns after a new thermistor, replace the PC Board.