Keston 64 Fault Code (Heat 2)

Safety disclaimer

Fault code summary

Manufacturer

Keston

Fault code

64

Fault type

Electrical

Severity

blocking

Reset

Not specified

Applicable models

Keston Heat 2 45, Keston Heat 2 55, Keston Heat 2 45P, Keston Heat 2 55P

Quick answer

Blocking due to mains undervoltage.

What does this fault code mean?

Blocking due to mains undervoltage.

Common causes

• Mains undervoltage.

Symptoms

• The boiler fault screen shows code 64 and a blocking condition.

What can the homeowner safely check?

• Check only whether the property has a wider electrical supply problem.

Boiler reset information

Reset information was not provided in the verified manufacturer source. 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

• Verify mains voltage and boiler power electronics.

Applicable boiler models

• Keston Heat 2 45
• Keston Heat 2 55
• Keston Heat 2 45P
• Keston Heat 2 55P

Related fault codes

• 1 — Heat 2

  Flow thermistor open circuit (blocking).

  Read fault code
• 10 — Heat 2

  Water-pressure sensor defect.

  Read fault code
• 11 — Heat 2

  Flow thermistor open circuit; lockout after 24 hours.

  Read fault code
• 12 — Heat 2

  Flow thermistor short circuit; lockout after 24 hours.

  Read fault code
• 13 — Heat 2

  Return thermistor open circuit; lockout after 24 hours.

  Read fault code
• 14 — Heat 2

  Return thermistor short circuit; lockout after 24 hours.

  Read fault code
• 19 — Heat 2

  Heat thermistor open circuit (blocking).

  Read fault code
• 2 — Heat 2

  Flow thermistor short circuit (blocking).

  Read fault code

Manufacturer documentation

FAULT FINDING MENU: Blocking due to mains undervoltage — code 64; refer to check mains supply.

Last verified

2026-09-12

Frequently asked questions