Ideal Boiler LF Fault Code: Ignition Failure Explained

What does the Ideal LF fault code mean?

The LF fault code on Ideal boilers indicates an ignition failure. The boiler has attempted to light the burner one or more times but has been unable to detect a stable flame, so it has shut itself down as a safety precaution. This could mean the burner failed to ignite at all, or it lit briefly then extinguished before the flame sensor could confirm a steady flame. The boiler enters lockout to prevent unburnt gas from accumulating or unsafe combustion from occurring. While the code itself is a protective feature rather than an immediate hazard, a persistent LF fault should be treated as urgent and investigated promptly.

Common causes

• Blocked or Frozen Condensate Pipe or Trap Common

  During colder months, the plastic condensate pipe that carries acidic water from the boiler to a drain can freeze solid where it runs outside. Internally, the condensate trap itself can become clogged with debris and sludge over time. In either case, the backed-up water interferes with safe combustion and triggers lockout. This is one of the most frequent causes of LF faults during UK winters.

• Dirty or Worn Ignition Electrode or Ionisation Probe Common

  The ignition electrode produces the spark to light the burner, while the ionisation probe confirms a flame is present. Over time, carbon deposits and ceramic-like fouling accumulate on both components, reducing their conductivity and reliability. The boiler may spark without lighting, or light briefly and then fail to confirm the flame, causing repeated lockouts. Electrode wear is accelerated in boilers that have not had regular annual servicing.

• Interrupted Gas Supply Common

  If the gas supply to the boiler is restricted or absent, no amount of sparking will produce a flame. Common culprits include a prepayment meter that has run out of credit, an isolation valve that has been accidentally turned off, or a wider supply interruption affecting the street. Other gas appliances in the home — such as the hob — will also fail if this is the cause.

• Faulty Spark Generator or HT Lead Sometimes

  The spark generator (sometimes called the ignition module) provides the high-voltage pulse that drives the electrode. If it is faulty or the high-tension lead connecting it to the electrode has deteriorated, the spark may be weak or absent entirely even though the electrode itself is undamaged. This is a relatively common cause in older installations.

• Fan or Flue Obstruction Sometimes

  The fan creates the draught needed to clear combustion gases and maintain the correct pressure conditions for safe ignition. A failing fan, a partially blocked flue terminal, or strong wind blowing back down the flue can all prevent the boiler from establishing the right conditions to light. The boiler's air pressure switch must confirm adequate airflow before it will attempt ignition.

• Faulty Gas Valve Sometimes

  The gas valve controls the flow of fuel to the burner. If it is sticking, failing to open fully, or opening erratically, the burner will receive insufficient or inconsistent gas — resulting in failed ignition or a flame that cannot sustain itself. Gas valve faults tend to appear in older boilers or those that have not been serviced regularly.

• PCB or Thermistor Fault Rare

  The printed circuit board (PCB) coordinates the entire ignition sequence. A fault on the PCB, or incorrect temperature signals from a faulty thermistor or external weather sensor, can cause the control logic to misfire the ignition sequence or misinterpret sensor readings, producing an LF lockout even when other components are functioning correctly.

How to fix it

1. Check that other gas appliances in the home are working DIY safe

   Turn on your gas hob or check another gas appliance. If nothing works, your gas supply may be interrupted. If you have a prepayment meter, check your credit balance and top up if needed. Locate the gas isolation valve — usually a yellow lever near the boiler or gas meter — and confirm it is turned to run parallel with the pipe (open position). If you suspect a gas supply failure affecting the whole street, contact your gas network operator on the free emergency line: 0800 111 999.

2. Inspect the condensate pipe for freezing or blockage DIY safe

   Find the condensate pipe — typically a white or grey plastic pipe, around 22–32 mm in diameter, that exits the boiler and either drops into an internal drain or runs outside the property. If the outdoor section feels solid or you can see ice, gently apply warm (not boiling) water from a kettle to the frozen section until it thaws and water flows freely. Never use a naked flame. Once thawed, proceed to reset the boiler.

3. Reset the boiler DIY safe

   Press and hold the reset button on the boiler's control panel (often marked with a flame symbol or the letter R) for around three seconds, then release. If there is no dedicated reset button, switch the boiler off at the fused spur, wait 30 seconds, and switch it back on. Allow the boiler a full minute to attempt ignition. Do not reset the boiler more than two or three times in quick succession — repeated resets without a successful ignition can trigger additional lockout codes and will not fix an underlying fault.

4. If the fault returns, stop resetting and call a Gas Safe registered engineer Gas Safe engineer

   If the LF code reappears after one or two resets, the cause is almost certainly something that requires professional diagnosis and repair. An engineer will check the ignition electrode and ionisation probe for wear and fouling, test the spark generator and HT lead, inspect the gas valve operation, verify fan performance and flue integrity, and check for PCB or sensor faults. Do not attempt to access internal gas components yourself — this is unsafe and, if your boiler is under warranty, will void it. Ensure the engineer you book is Gas Safe registered (you can verify this at gassaferegister.co.uk).

Parts you may need

• Ignition electrode · from £25
• Ionisation probe (flame sensor) · from £20
• Spark generator / ignition module · from £55
• HT (high-tension) ignition lead · from £15
• Condensate trap · from £30
• Gas valve · from £150

The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.

Typical repair cost

Expect to pay roughly £150–£320, depending on the underlying cause.

Frequently asked questions