Verified boiler fault code

Keston 20 Fault Code (Heat 2)

Heat thermistor short circuit (blocking).

Safety disclaimer

Boiler repair help from Aza & Sons Services

Fault code summary

Manufacturer
Keston
Fault code
20
Fault type
Temperature sensor, Overheating
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

Heat thermistor short circuit (blocking).

What does this fault code mean?

Heat thermistor short circuit (blocking).

Common causes

  • Heat thermistor short circuit.

Symptoms

  • The boiler fault screen shows code 20 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 heat thermistor and its wiring.

Applicable boiler models

Related fault codes

Manufacturer documentation

Keston Heat 2 Installation & Servicing Manual

FAULT FINDING MENU: Heat thermistor short circuit (blocking) — code 20.

Last verified

2026-09-12

Frequently asked questions

What does Keston 20 mean?

Heat thermistor short circuit (blocking).

Can I reset a Keston 20 fault?

The Heat 2 fault screen shows a Press Reset prompt; do not repeatedly reset an unresolved fault.