Ideal L4 Fault Code (Logic Combi)

Safety disclaimer

Fault code summary

Manufacturer

Ideal

Fault code

L4

Fault type

Temperature sensor, Central heating

Severity

Not specified

Reset

Ideal recommends restarting the appliance once; do not repeatedly restart an unresolved fault.

Applicable models

Logic Combi 24, Logic Combi 30, Logic Combi 35

Quick answer

Flow thermistor fault.

What does this fault code mean?

Flow thermistor fault.

Common causes

• The flow thermistor is associated with maintaining boiler temperature and is reporting a fault.

Symptoms

• The boiler displays L4.

What can the homeowner safely check?

• Ideal's guide recommends restarting the appliance; contact Ideal or a Gas Safe registered engineer if the code remains.

Boiler reset information

Ideal recommends restarting the appliance once; do not repeatedly restart 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

• Assess the flow thermistor and its circuit.

Applicable boiler models

• Logic Combi 24
• Logic Combi 30
• Logic Combi 35

Related fault codes

• F1 — Logic Combi

  Low water pressure.

  Read fault code
• F2 — Logic Combi

  Flame loss.

  Read fault code
• F3 — Logic Combi

  Fan fault.

  Read fault code
• F4 — Logic Combi

  Flow thermistor fault.

  Read fault code

Manufacturer documentation

‘F4’ / ‘L4’ on Your Ideal Logic Combi — Flow Thermistor — we recommend restarting your appliance if you see this fault code.

Last verified

2026-09-12

Frequently asked questions