Overview
The L4 or F4 error on Ideal Logic Max & Logic Plus boilers means the boiler’s flow thermistor (flow temperature sensor) or its control circuit has reported a fault. The thermistor measures the flow pipe temperature so the boiler can control burner output and protect against overheating or dry firing. The code can appear if the sensor itself is faulty, its wiring or connector is damaged or disconnected, the sensor is not correctly clipped to the flow pipe, or if the control PCB is detecting an implausible reading. It can also be triggered indirectly by poor or no water circulation (airlock, closed valves, stuck pump) which makes the sensor reading incorrect for the operating conditions. Severity is moderate: the boiler will usually lock out or refuse to run the central heating until the fault is cleared. That prevents potential damage but leaves you without heating (and possibly hot water depending on model). Initial checks and simple fixes (re-pressurising, resetting, opening valves, bleeding radiators) are often possible for a competent homeowner. However, diagnosing and repairing thermistor wiring, replacing sensors, or repairing PCB and pump faults requires a Gas Safe registered engineer and electrical/gas safety expertise, so professional attendance is normally needed if basic checks don’t clear the code.
Troubleshooting Steps
Safety precautions:
- If you are unsure or not competent working around gas appliances, stop and call a Gas Safe engineer. Improper repairs can be dangerous.
- Before opening the boiler casing switch the boiler off at the mains isolator and let the unit cool. Be aware of hot surfaces and hot water in the system.
- Do not attempt to alter gas valves, seals or internal gas components.
Initial checks homeowners can do:
1. Note the exact code and any accompanying behavior (locked out, flashing indicators). Try a simple reset using the boiler RESTART button or switching the power off/on at the isolator. If the code returns immediately, continue checks.
2. Check system pressure on the boiler gauge. If pressure is low (below about 1.0 bar on a Logic System or around the installed pre-pressure), repressurise the system using the filling loop to around 1.0–1.5 bar per your manual, then reset the boiler.
3. Ensure central heating and boiler controls are calling for heat (timer/stat set correctly) so the boiler is in the expected mode when you test.
4. Check radiators and pipework: are radiator valves open, is there any obvious air in the system? Bleed a few radiators to remove air and then top up pressure and reset.
5. Listen for the pump when the boiler tries to fire (a faint hum/vibration). If the pump is dead, the system may not circulate and will present as a no-flow/sensor fault.
Specific diagnostic and fix steps (do not open the boiler unless competent):
1. Try reset + repressurise: many flow thermistor/no-flow faults are caused by low pressure or airlocks. Re-pressurise to the correct pressure, bleed radiators, then reset. If this clears the code and the boiler runs normally, monitor pressure and performance.
2. Check visible external valves: confirm any isolating valves on the flow/return and the filling loop are in the correct (open) positions. Check motorized zone valves are not stuck mid-position and that room thermostat is not rapidly switching.
3. Check for obvious pump problems: with the boiler powered and calling for CH, carefully feel the pump housing for vibration/heat (do not touch moving parts). If pump is cold and silent when it should be running, a pump fault or wiring fault is likely and requires an engineer.
4. Visual inspection of sensor location: on Logic boilers the flow thermistor is typically clipped to the flow pipe inside the boiler case. Switch power off and, only if you are competent, remove the case to visually check that the sensor is correctly clipped to the flow pipe and the connector is secure (no obvious corrosion or damaged wires). Do not force connectors or touch live wiring. If the sensor is loose, re-seating it correctly can sometimes clear the fault.
5. If the sensor wiring looks damaged, or the sensor is missing or broken, do not attempt to replace it yourself unless you are a qualified engineer. Electrical and plumbing work inside a boiler should be done by a Gas Safe engineer.
6. If you have recently had work on the heating system (pump replacement, cylinder work, new wiring, weather compensation fitted), mention this to the engineer as incorrect wiring or an incorrectly configured PCB/boiler chip card can cause thermistor errors.
7. If none of the above clears the fault or if the code points to a PCB or intermittent sensor reading (fault returns after reset), arrange a Gas Safe registered engineer visit. The engineer will test thermistor resistance, check wiring continuity, verify correct sensor clamp placement, test pump operation and, if necessary, replace the thermistor or repair/replace the control PCB.
When to call a professional:
- Call a Gas Safe engineer if resetting and basic system checks (pressure, valves, bleeding radiators) do not clear the code.
- Call an engineer for any internal checks that require opening the boiler, testing or replacing sensors, repairing wiring, replacing the pump, or replacing the PCB.
- If you suspect gas supply issues, flame loss or any other hazardous condition, isolate the boiler and call a Gas Safe engineer immediately.
Summary: Start with safe, low-risk checks—reset, check and top up pressure, bleed radiators, ensure valves are open and the pump runs. If the fault persists, or if you find damaged sensor wiring or failed components, stop and get a Gas Safe registered engineer to diagnose and repair. Do not attempt internal electrical or gas repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Logic Max & Logic Plus Regular Boiler.