Verified boiler fault code
Keston 3 Fault Code (Heat 2)
Return thermistor open circuit (blocking).
Safety disclaimer
Fault code summary
- Manufacturer
- Keston
- Fault code
- 3
- Fault type
- Temperature sensor, Return sensor
- Severity
- blocking
- Reset
- The Heat 2 fault screen shows a Press Reset prompt; do not repeatedly reset an unresolved fault.
- Applicable models
- Keston Heat 2 45, Keston Heat 2 55, Keston Heat 2 45P, Keston Heat 2 55P
Quick answer
Return thermistor open circuit (blocking).
What does this fault code mean?
Return thermistor open circuit (blocking).
Common causes
- Return thermistor open circuit.
Symptoms
- The boiler fault screen shows code 3 and a blocking condition.
What can the homeowner safely check?
- Record the code and contact a qualified engineer.
Boiler reset information
The Heat 2 fault screen shows a Press Reset prompt; do not repeatedly reset an unresolved fault.
When should I call an engineer?
Contact a suitably qualified heating engineer when the fault persists, the boiler locks out, or the manufacturer instructions do not resolve it. For gas appliance work in the UK, use a Gas Safe registered engineer.
Engineer diagnosis
- Test the return thermistor and its wiring.
Applicable boiler models
Related fault codes
- 1 — Heat 2
Flow thermistor open circuit (blocking).
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Heat thermistor open circuit (blocking).
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Manufacturer documentation
Keston Heat 2 Installation & Servicing ManualFAULT FINDING MENU: Return thermistor open circuit (blocking) — code 3.
Last verified
2026-09-12
Frequently asked questions
What does Keston 3 mean?
Return thermistor open circuit (blocking).
Can I reset a Keston 3 fault?
The Heat 2 fault screen shows a Press Reset prompt; do not repeatedly reset an unresolved fault.