Baxi H.03-.02 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi H.03-.02 fault code mean?
The H.03-.02 is a temporary (H-type) blocking fault on Baxi boilers, indicating that the flame supervision circuit failed to detect or maintain an ionisation current during operation. In plain terms, the boiler either attempted to fire and could not confirm a stable flame, or the flame dropped out during normal running. As a blocking fault rather than a hard lockout, the boiler may attempt to recover automatically once the underlying condition clears — but if the problem persists or repeats, manual intervention is needed. The root cause typically lies somewhere in the ignition and flame-sensing chain: the ionisation electrode, gas supply, ignition leads, gas valve, or flue system.
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
- Faulty or sooty flame-sensing electrode Common
The ionisation electrode sits in the combustion chamber and confirms a flame is present by detecting a small electrical current through the flame. Over time it accumulates carbon deposits or soot, which insulates the tip and prevents a reliable reading. This is the single most common cause of H.03-.02 appearing, particularly on boilers that have not been serviced recently.
- Gas supply interruption Common
If gas pressure to the property drops — due to a closed isolation valve, works on the local network, or a prepayment meter running out — the burner cannot sustain a flame and this code will appear. It is always worth checking whether other gas appliances such as a hob or gas fire are working before assuming the boiler itself is at fault.
- Dirty or damaged ignition electrode or leads Sometimes
The ignition electrode generates the spark that lights the burner, and its connecting lead carries the high-voltage signal from the PCB. Cracked ceramic insulation, carbon tracking, or a loose terminal can prevent the spark from firing reliably, causing intermittent or complete ignition failure that the flame sensor then cannot confirm.
- Faulty or sticking gas valve Sometimes
The gas valve meters fuel to the burner under instruction from the PCB. If the valve is slow to open, partially blocked, or beginning to fail electrically, insufficient gas reaches the burner and the flame either fails to establish or drops out shortly after lighting. Repeated H.03-.02 faults with no obvious electrode or supply issue point strongly towards the gas valve.
- Flue obstruction or terminal blockage Sometimes
A partially blocked flue restricts the combustion air supply and exhaust gas removal. Poor combustion as a result can destabilise the flame to the point where the ionisation electrode can no longer detect it. Bird nests, debris at the terminal, or a damaged flue section are typical causes, especially after winter or in properties with exposed roof-level terminals.
- PCB (printed circuit board) fault Rare
The PCB processes the signal from the ionisation electrode and controls spark timing via the ignition module. If the board develops a fault — through moisture ingress, power surge, or component ageing — it may misinterpret the electrode signal or fail to generate ignition reliably, producing a flame loss code even when all physical components are in order.
How to fix it
- Check that your gas supply is live DIY safe
Try a gas hob ring or gas fire. If these are also not working, the issue is almost certainly with the gas supply to your property rather than the boiler itself. Contact your gas supplier or check whether a gas isolation valve near the meter has been accidentally closed.
- Reset the boiler DIY safe
Locate the reset button on the front of your Baxi boiler — typically a symbol resembling a flame with a line through it or a recessed button. Press and hold for approximately two seconds until the boiler restarts. Allow a full heating cycle to complete and monitor whether the fault reappears. Do not reset more than two or three times; repeated resets without resolution will not fix the underlying problem and could mask a more serious issue.
- Monitor whether the fault clears on its own DIY safe
Because H.03-.02 is a temporary blocking fault, the boiler may self-recover once the momentary condition — for example, a brief gas pressure dip — has passed. If the fault appeared once and the boiler is now running normally after a reset, note the date and keep an eye on it. A single isolated occurrence is less concerning than a code that returns repeatedly.
- Have a Gas Safe engineer inspect and clean the flame-sensing electrode Gas Safe engineer
An engineer will remove the electrode from the combustion chamber, examine it for carbon deposits, cracking, or tip erosion, and clean it with a fine abrasive or replace it if worn. They will also verify the electrode gap meets the manufacturer's specification and check the connecting lead for continuity and secure termination at the PCB.
- Have the ignition electrode, lead, and connections checked Gas Safe engineer
The engineer will inspect the ignition electrode ceramic for cracking, test the high-tension lead for breakdown, and confirm the spark generator on the PCB is firing correctly. Loose terminal connections at either end of the lead are a common and easily overlooked cause of intermittent ignition failure.
- Have the gas valve inspected or replaced if necessary Gas Safe engineer
If the electrode and ignition system are in good order, the engineer will test the gas valve for correct opening speed, gas pressure at the injector, and electrical operation. A valve that is sticking or delivering insufficient gas flow will need to be replaced — this is gas work and must only be carried out by a Gas Safe registered engineer.
- Have the flue and air supply terminal checked Gas Safe engineer
The engineer will inspect the entire flue run for obstructions, damage, or incorrect fall, and check the external terminal is clear of debris. They will also confirm adequate combustion air is available to the appliance, particularly in well-sealed properties where air supply may have been inadvertently restricted.
- Have the PCB tested or replaced if all other components are in order Gas Safe engineer
If the electrode, gas valve, and flue all check out correctly, the engineer will test the PCB's flame-sensing circuit and ignition output. PCB replacement is typically a last resort given its cost, but is occasionally the underlying cause of persistent flame loss codes.
- Call a Gas Safe registered engineer if the fault persists or returns repeatedly Gas Safe engineer
If the fault reappears after a reset, or has occurred more than twice in a short period, contact a Gas Safe registered engineer to carry out a full diagnostic inspection. Continuing to run a boiler that is repeatedly losing flame without investigation is unsafe and could indicate a serious issue with the gas valve or combustion system.
Parts you may need
- Ionisation / flame-sensing electrode · from £25
- Ignition electrode · from £22
- High-tension ignition lead · from £18
- Gas valve · from £130
- PCB (printed circuit board) · from £220
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 £100–£320, depending on the underlying cause.
Frequently asked questions
Is H.03-.02 a lockout or will my Baxi boiler restart on its own?
H.03-.02 is classed as a temporary or blocking fault, which means the boiler pauses for safety but may attempt to restart automatically once the issue that caused the flame loss has resolved. If the boiler has locked out completely and will not restart without a manual reset, press the reset button once or twice and allow a full cycle. If the fault returns persistently, that suggests a component problem rather than a one-off event, and a Gas Safe engineer should investigate.
Can I clean the flame sensor myself to fix this fault?
The flame-sensing (ionisation) electrode is located inside the combustion chamber, directly in the gas pathway of your boiler. Accessing it involves removing the boiler casing and working in close proximity to the burner and gas components. This is not a task recommended for homeowners — it should be carried out by a Gas Safe registered engineer who can also verify the electrode gap and condition of the ignition system while they are there.
How much does it typically cost to fix a Baxi H.03-.02 fault?
For the majority of people, the repair involves cleaning or replacing the flame-sensing electrode or ignition components, which typically costs between £100 and £200 including labour. If the gas valve needs replacing, expect to pay in the region of £250–£350 all in. PCB replacement is less common but significantly more expensive — often £400–£700 depending on parts availability and labour time — and is worth weighing against the age and overall condition of the boiler before proceeding.
Why does my Baxi keep showing H.03-.02 after I reset it?
A fault that clears on reset but returns within hours or days almost always indicates a physical component is degrading rather than a one-off blip. The most likely culprits are a flame-sensing electrode that is worn or coated in deposits, an ignition lead that is breaking down intermittently, or a gas valve that is slow to open under certain conditions. An annual service would likely have caught these issues early; if your boiler has not been serviced for over a year, booking one now is a sensible first step alongside the fault investigation.