393 Outdoor temperature sensor fault

What the control is telling you

Fault detected in the outdoor (outdoor reset) temperature thermistor.

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

Likely causes

• Loose, broken or damaged sensor wiring
• Sensor unplugged (reads -40)
• Parameter 00 set wrong for sensor use
• Sensor wired to wrong boiler terminals
• Sensor buried by snow or in direct sunlight
• Failed outdoor sensor

Test and repair steps

1. Inspect the sensor wiring from the outdoor 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 31 and compare with a thermometer at the same location. Within 5° = OK; more than 5° off = continue; -40 = unplugged or poor connection.
4. Check Parameter 00 factory setting A = sensor enabled; set to b only if no outdoor sensor is used (Rinnai strongly recommends using it).
5. Confirm the sensor is wired to the correct boiler terminals (see wiring diagram in the source manual, p.26) and the run has no breaks or loose joints.
6. Mount the sensor well above expected snow levels and on the north side of the building, out of direct sun; relocate if needed.
7. Measure sensor resistance on a 20K ohm range and compare against the outdoor thermistor table in the source manual (p.27).
8. Cross-check with the controller's performance data; if correct, reinstall the sensor.
9. Call for heat; if code 393 comes back, replace the outdoor sensor.