Verified boiler fault code
Keston 42 Fault Code (Heat 2)
Blocking due to heat-exchanger overheat.
Safety disclaimer
Fault code summary
- Manufacturer
- Keston
- Fault code
- 42
- Fault type
- Overheating, Heat exchanger
- 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
Blocking due to heat-exchanger overheat.
What does this fault code mean?
Blocking due to heat-exchanger overheat.
Common causes
- Heat-exchanger overheat.
Symptoms
- The boiler fault screen shows code 42 and a blocking condition.
What can the homeowner safely check?
- Record the code and allow the appliance to cool; 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
- Investigate heat-exchanger temperature and primary circulation.
Applicable boiler models
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Manufacturer documentation
Keston Heat 2 Installation & Servicing ManualFAULT FINDING MENU: Blocking due to heat exchanger overheat — code 42.
Last verified
2026-09-12
Frequently asked questions
What does Keston 42 mean?
Blocking due to heat-exchanger overheat.
Can I reset a Keston 42 fault?
The Heat 2 fault screen shows a Press Reset prompt; do not repeatedly reset an unresolved fault.