Ideal LC Fault Code: Causes, Fixes & Repair Costs
What does the Ideal LC fault code mean?
The LC fault code (also shown as L5 on some Ideal models) appears when your boiler has locked out and been manually reset five or more times within a 15-minute window. Ideal boilers use 'L' prefix codes to indicate lockout conditions, and LC is essentially the boiler's built-in protection against repeated failed restart attempts. Crucially, LC is not the root problem itself — it is a secondary response that tells you something else is persistently wrong. The boiler keeps failing, someone keeps hitting reset, and after the fifth attempt in quick succession the LC code is triggered to prevent further cycling. To clear LC you must identify and fix the underlying fault first; simply resetting again will only retrip the code.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Repeated manual resets masking an underlying fault Common
The most direct cause of LC appearing is someone pressing the reset button repeatedly without addressing why the boiler keeps locking out. Each lockout points to a fault that has not been resolved. Once the reset counter hits five within 15 minutes, LC is displayed. Stop resetting and investigate the root cause.
- Low system pressure causing repeated lockouts Common
If the boiler's water pressure has dropped below approximately 0.5 bar, the boiler will lock out on a low-pressure fault. If you reset it without topping the pressure back up, it locks out again — and again — until LC appears. Check the pressure gauge on the boiler front; it should read between 1.0 and 1.5 bar when cold.
- Frozen or blocked condensate pipe Common
In cold weather, the condensate pipe — the white or grey plastic pipe that runs from the boiler to an outside drain — can freeze solid. When it does, the boiler shuts down to protect itself. If this happens and you keep resetting without thawing the pipe, LC will quickly follow. This is a very common winter fault.
- Ignition or flame detection failure Common
Worn or dirty ignition electrodes, a faulty flame sensor, or loose wiring can prevent the boiler from establishing or confirming a flame. The boiler fires, fails to detect ignition, locks out, and the cycle repeats. This is one of the more frequent underlying causes referred to a Gas Safe engineer.
- Gas supply interruption or low gas pressure Sometimes
If other gas appliances in the property are also not working, the supply to the property may be interrupted. If gas supply appears normal elsewhere but the boiler still won't fire, internal gas pressure or valve issues may be present — these require engineer investigation.
- Faulty PCB Rare
The printed circuit board controls the boiler's operation and monitors all sensors. A PCB fault can cause the boiler to misread conditions and lock out repeatedly for no apparent external reason. PCB faults are relatively uncommon but cannot be ruled out, especially on older units.
How to fix it
- Stop resetting the boiler immediately DIY safe
Continuing to press reset will only re-trigger the LC code. The boiler has locked out multiple times for a reason — repeated resets without fixing the underlying fault will not help and may mask important diagnostic information.
- Power cycle the boiler at the wall switch DIY safe
Rather than using the boiler's own reset button, turn the boiler off at the mains switch, wait at least two minutes, then switch it back on. This clears the LC counter and gives the boiler a clean start. Do this only once before checking the items below.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, use the filling loop (usually a braided flexible hose or lever valve beneath the boiler) to top up slowly until the gauge reaches around 1.2 bar, then close the filling loop securely. If you are unsure how to do this, your boiler manual will have a diagram.
- Check your gas supply DIY safe
Confirm that other gas appliances in your home — such as your gas hob or fire — are working normally. If nothing gas-powered is working, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999. If gas supply is fine elsewhere, the issue is specific to the boiler.
- Thaw a frozen condensate pipe if cold weather is likely the cause DIY safe
If outdoor temperatures are near or below freezing and the LC code appeared overnight, locate the condensate pipe outside — it is typically a white or grey plastic pipe about 22 mm in diameter running from the boiler through an external wall to a drain. Pour warm (not boiling) water slowly along its full length, or wrap a warm (not hot) cloth around it. Listen for a gurgling sound inside the boiler, which indicates the blockage has cleared. Once thawed, reset the boiler once.
- Attempt a single reset and observe the boiler DIY safe
After completing the checks above, press the reset button once. Watch and listen as the boiler attempts to fire. If it lights and runs normally, monitor it over the next hour. If it locks out again within minutes, do not reset it again — this confirms an underlying fault that requires professional diagnosis.
- Call a Gas Safe registered engineer to diagnose and repair the root cause Gas Safe engineer
If the boiler locks out again after your single reset, an engineer is needed. They will use diagnostic tools to identify the actual fault — which may involve testing the ignition electrodes and flame sensor, inspecting the gas valve and internal gas pressure, or testing the PCB. All work involving internal gas components must legally be carried out by a Gas Safe registered engineer. Ask to see their Gas Safe ID card before any work begins.
Parts you may need
- Ignition electrodes · from £25
- Flame sensor (ionisation electrode) · from £20
- Gas valve · from £120
- PCB (printed circuit board) · from £180
- Condensate pipe insulation wrap · from £15
- Pressure relief valve · from £30
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–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between LC and L5 on an Ideal boiler?
On many Ideal boiler models these two codes are essentially the same fault presented differently depending on the model variant or firmware version. Both indicate that the boiler has locked out and been reset too many times in a short period — typically five resets within 15 minutes. If you see either code, the advice is the same: stop resetting, power cycle, check pressure and condensate, then call an engineer if the fault persists.
Can I fix the LC fault myself?
You can carry out a few safe checks yourself: power cycling the boiler, topping up system pressure via the filling loop, and thawing a frozen condensate pipe. However, if any of these do not resolve the underlying lockout, a Gas Safe registered engineer must carry out further investigation. Working on the gas valve, ignition system, PCB, or any internal component is a legal requirement for Gas Safe registered engineers only.
Why does my Ideal boiler keep locking out and triggering LC?
The LC code appears because the boiler locked out repeatedly — not because LC is a fault in its own right. The most common reasons for repeated lockouts are low system pressure, a frozen condensate pipe in cold weather, ignition or flame detection problems, and occasionally gas supply issues or a faulty PCB. A Gas Safe engineer will be able to pinpoint the exact cause with diagnostic equipment.
How much will it cost to fix the fault behind an LC code?
Because LC is a secondary code, the repair cost depends entirely on what the underlying fault turns out to be. Most repairs — ignition electrodes, flame sensor replacement, or a straightforward gas valve issue — typically fall in the £150 to £380 range including labour. A PCB replacement is one of the more expensive outcomes and can push costs to £450 or more; if your boiler is older, it may be worth considering a replacement boiler at that point. Always get a written quote before any work begins.