Safety Protocol & Classification
Safety Classification: CALL-A-PRO
Homeowner DIY Checks Allowed: NO — Professional Technician Required
What This Symptom Means
The boiler electronic control board has detected an open circuit, short circuit, or out-of-range signal from a temperature thermistor (NTC).
Likely Causes
- Defective supply, return, DHW, or outdoor reset temperature thermistor.
- Corroded, loose, or damaged wiring harness connections between sensor and board.
- Sensor wire shorted against metal casing or hot piping.
- Moisture ingress into sensor housing.
Homeowner Inspection Boundaries
Because this symptom directly involves sealed pressure vessels, gas combustion, high-voltage circuits, or boiler safety limit devices, homeowner DIY disassembly or internal testing is not permitted under Hydronic Authority safety guidelines. Homeowners should observe external indicators (such as display codes or pressure gauges) and contact a certified hydronic technician.
What NOT To Do
- Do not attempt to splice or bridge sensor wiring without correct resistance testing specifications.
- Do not replace sensors while system is hot and pressurized.
When to Call a Professional
Temperature sensor diagnosis requires multimeter resistance testing against manufacturer temperature-resistance tables. Call a pro.
Error Code Cross-Reference
If your boiler display is showing an error code or fault message associated with this symptom, look up the code in the Phillips Hydronics Error Code Library for exact manufacturer actions.
Related Equipment & Component Guides
Sources & Documentation
- error-codes.json — Compiled across 12 boiler brand fault code tables
- hydronic-authority/boiler-troubleshooting.html —