Is this tool actually free?

Yes. No signup, no credit card, no paywall. We ask for your email so we can send the checklist — that is the only exchange.

Which boiler brands are supported?

Every major UK brand — Worcester Bosch, Vaillant, Baxi, Ideal, Viessmann, Glow-worm, Potterton, Ferroli, Ariston, Alpha, Main, Keston, and more. The AI also handles air source heat pumps from Daikin, Mitsubishi, Samsung and LG.

Is the advice Gas Safe compliant?

The checklist never recommends work outside Gas Safe scope. It is structured reference information to speed up your own diagnostic process. Always follow manufacturer documentation and your own competence.

How accurate is the AI?

The AI is trained on manufacturer documentation and common field-engineering knowledge. For fault codes with a single well-documented cause it is very accurate. For vague symptoms with multiple potential causes it will suggest the most likely checks in order — you decide what actually applies.

What data do you store?

Your email, the appliance details you entered, and the generated checklist. If you tick the "publish anonymously" box, we publish the checklist to a public URL with the make, model and symptoms — but never your email or any personal information.

Why do you ask to publish it publicly?

Other engineers searching Google for the same fault benefit from your submission. It is the Stack Overflow model — the community gets stronger with each contribution. You can untick the box if you prefer to keep it private.

Is there a limit on how many I can generate?

To keep the tool free and prevent abuse, we limit submissions to a few per hour per user. If you hit the limit, just wait an hour and try again.

No. We only email the checklist you asked for. If you tick the marketing checkbox we may send the occasional BoilerTag tip or product update — unsubscribe any time.

Free AI tool for engineers

AI Boiler Fault Diagnostic Checklist

Stuck on a fault? Enter the make, model, and fault code — we'll email you a structured checklist you can work through on-site in under 30 seconds.

15 real-world fault checklists published · 6 boiler makes covered

How it works

1
Enter the boiler details
Make, model, fault code if you have one, and a quick description of the symptoms.
2
Get your checklist
A step-by-step diagnostic list ordered by safety, cost-to-check, and likelihood.
3
Email copy sent
We email you the full checklist so you can refer back to it on-site or in the van.

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Frequently asked questions

Recently published diagnostics

Browse all →

Every checklist submitted by an engineer (with consent) becomes a public reference page. Here are the 20 most recent.

Ideal Logic
The Ideal Logic + Heat boiler failing to ignite, despite new spark generator and electrode, 37mbar standing pressure dropping to 34mbar on ignition attempt, and 233V DC to the gas valve, points towards a persistent ignition lockout. Common causes for ignition lockout (often indicated by an L2 fault code on Ideal Logic boilers) include issues with gas supply, gas valve, flue/air intake, condensate, flame rectification, or the PCB. Given the steps already taken, further investigation into gas valve resistance, flame rectification, and potential PCB faults is crucial.
Intergas Compact Hre 24 — 5
Intergas Compact HRE 24/18 fault code 5 indicates a 'poor flame signal' or 'weak ignition flame signal'. This means the boiler is attempting to ignite but the flame signal detected is too weak or unstable, leading to the boiler shutting down as a safety precaution. This is a common issue that often points to a structural problem rather than a temporary fault, and typically requires a Gas Safe registered engineer to diagnose and repair.
Ideal Logic Max Combi C24 — Overheat Lockout 1/4
The Ideal Logic MAX Combi C24 is displaying an 'Overheat Lockout' fault (sometimes referred to as L1), with no heating or hot water and the pressure gauge at 0 bar. This fault indicates the boiler temperature has exceeded safe operating limits, shutting down as a safety precaution. Common causes include low water pressure, poor circulation, air in the system, a blocked heat exchanger, or closed valves. The 0 bar pressure reading is a critical factor, as the boiler will not operate below 0.3 bar. There is also a safety recall for some Ideal Logic 2 Combi and System Boilers (including MAX models) regarding a risk of fire due to dry operation. It is crucial to check if the specific boiler is affected by this recall.
Vaillant Ecotec Plus 610 — F.028
The Vaillant ecoTEC plus 610 displaying fault code F.028 indicates an ignition failure, meaning the boiler failed to ignite or detect a flame during startup. This leads to the boiler locking out as a safety measure. Common causes include issues with the gas supply, a blocked or frozen condensate pipe, a faulty ignition system (electrode, lead, transformer), or a defective gas valve or PCB. The clicking sound before lockout is consistent with the boiler attempting ignition. The normal pressure reading suggests the issue isn't related to low system pressure.
Worcester Greenstar 12i System — Heat exchanger overheat
The Worcester Greenstar 12i System displaying a "Heat exchanger overheat" fault code indicates the boiler's safety systems have activated due to excessively high temperatures within the heat exchanger. This is a critical safety lockout designed to prevent damage to the appliance. Common causes include poor water circulation, a blocked heat exchanger (often due to limescale or sludge), airlocks, or a faulty pump. The intermittent nature suggests a developing issue rather than a sudden component failure. The pressure reading of 1.5 bar is within the normal operating range, so low pressure is unlikely to be the primary cause.
Vaillant Ecotec Plus 610 — F.040
The Vaillant ecoTEC plus 610 displaying fault code F.040 indicates that the air ratio is too low, often due to a fault in the flue gas route, recirculation, or a blockage. It can also be caused by a blocked air intake pipe or condensate in the combustion chamber due to a blocked condensate discharge. The symptoms of the fan kicking in, ignition sequence attempting, but locking out before ignition, align with this fault code, suggesting an issue preventing proper combustion. Given the intermittent nature and occurrence when firing from cold, it points towards a component that might be struggling to perform correctly under initial demand. The gas tightness check and clear flue terminal/condensate trap are good initial steps, but further investigation into the air/flue system and ignition components is necessary.
Glow Worm Energy 25c — F.32
The Glow Worm Energy 25c displaying fault code F.32, with the fan cycling erratically and making a struggling humming noise, indicates an 'air pressure issue' or 'fan speed outside the tolerance values'. This is often caused by a fan malfunction, blocked air intake/flue, or a faulty Hall sensor. The intermittent nature becoming constant suggests a component is failing completely.
Worcester Bosch Greenstar
Frequent pressure drops on a Worcester Bosch Greenstar boiler typically indicate a leak within the sealed system, a faulty pressure relief valve (PRV), or an issue with the expansion vessel. While repressurising the system can offer a temporary fix, persistent drops require a thorough diagnosis by a Gas Safe registered engineer to identify and rectify the underlying cause.
Glow Worm Energy 25c — F.28
The Glow Worm Energy 25c displaying an F.28 fault code indicates an ignition failure. This means the boiler is attempting to ignite but is unable to establish a flame. The clicking noise you hear is likely the boiler trying to spark. Common causes range from simple gas supply issues to more complex component failures within the ignition system. The pressure gauge reading of 1 bar is within the normal cold operating range, so low pressure is unlikely to be the primary cause of this specific fault.
Baxi 824 Combi 2 — E.04.10
The Baxi 824 Combi 2 displaying fault code E.04.10 indicates that the burner has failed to ignite after four attempts. This is a common fault that can be caused by several issues, including a gas supply problem, a frozen condensate pipe, or a fault with the ignition electrodes or flame sensor. The boiler is designed to lock out for safety reasons when this fault occurs. The pressure gauge reading of 1.5 bar is normal, so low system pressure is unlikely to be the cause in this instance.
Baxi 800
The Baxi 800 boiler intermittently heating without a fault code can be attributed to several common issues, including low system pressure, circulation problems, or issues with the thermostat or diverter valve. While no specific fault code was provided, common Baxi fault codes related to heating issues include E119 (low pressure) and E125 (circulation fault). It's crucial to address these systematically, prioritising safety and checking the simplest causes first.
Worcester Greenstar Cdi
The Worcester Greenstar CDi is a condensing combination boiler. The symptom of 'won't come up to heat' without a specific fault code can indicate several issues, ranging from basic user settings to component failures. Common causes include low system pressure, issues with the gas supply, condensate blockages, or problems with key components like the fan, ignition electrodes, or PCB. There are no specific service bulletins for a general 'no heat' fault, but manufacturer documentation and reputable trade sources highlight these common failure modes.
Baxi 100he — Ignition Lockout
The Baxi 100HE displaying an 'Ignition Lockout' fault, with no heating or hot water and no noises, indicates the boiler has failed to ignite after several attempts and has entered a safety lockout state. This is a common fault for Baxi 100HE models, particularly after cold weather, and can be caused by various issues including a blocked condensate trap, faulty ignition components, or issues with the gas supply or fan operation.
Ideal Vogue Max Combi 26 — IGNITION LOCKOUT
The Ideal Vogue Max Combi 26 displaying 'IGNITION LOCKOUT' indicates that the boiler has failed to ignite after several attempts and has entered a safety lockout mode. This is a common safety feature to prevent dangerous situations. The boiler will not make any noises, provide hot water, or heating until the fault is cleared and the system is reset. Common causes include issues with the gas supply, a blocked condensate pipe, or faulty ignition components.
Worcester Greenstar 12i System — Ignition lockout
The Worcester Greenstar 12i System displaying an 'Ignition lockout' fault indicates that the boiler has attempted to ignite but failed to detect a flame, leading to a safety shutdown. This is a protective mechanism to prevent unsafe operation. The boiler will not restart until the underlying fault is identified and resolved, and the boiler is manually reset. Repeatedly resetting the boiler without addressing the cause can lead to further damage to components like the PCB and ignition parts.