If your Baxi 800 Combi 2 shows H.03.05, Baxi’s current error-code table defines it as “Power supply voltage too low.” The boiler has detected that the electrical supply available to it is below the level it expects.
This is not the same as a complete power cut. The display may still be visible, yet the boiler may pause or restrict operation because the detected voltage is too low for reliable running. The code does not establish whether the source lies with the wider electricity supply, the property’s electrical installation, a temporary power arrangement or the boiler itself.
What does Baxi H.03.05 mean?
Voltage is the electrical pressure that allows equipment to operate. A gas boiler still needs electricity for its controls, display, pump and safety systems. H.03.05 means the boiler has detected a low-voltage condition at its supply.
The display is useful evidence, not a complete diagnosis. It cannot safely tell a homeowner which cable, connection or component is responsible. Testing mains electricity, opening a fused connection unit, removing the boiler casing or probing internal wiring is not a homeowner task.
What might happen when the voltage is low?
The boiler may stop firing, fail to respond normally to a demand, restart unexpectedly when the supply recovers or show the code intermittently. Heating and hot water may both be affected because a combi boiler provides both directly from the same appliance.
A temporary dip can coincide with a local supply disturbance, but the timing alone does not prove the cause. Equally, other lights or appliances appearing normal does not rule out the condition the boiler detected.
Safe checks you can make without touching the electrics
Keep your checks observational. Look at the boiler display and ordinary household conditions from a safe position. Useful details include:
- Whether there has just been a power cut, flicker or local electricity interruption.
- Whether clocks on other appliances reset at about the same time.
- Whether the code appears continuously or only at certain times.
- Whether heating, hot water or both are unavailable.
- Whether the boiler display goes blank, dims or restarts.
- Whether neighbours report a wider supply problem.
- Whether the property is using any temporary generator, inverter or backup-power equipment.
Record these points for the professional handling the fault. Do not move the boiler’s electrical supply to another socket, use an extension lead, open the consumer unit or try to measure voltage yourself.
Should you reset the boiler?
If the wider electricity supply has clearly been interrupted, wait for it to become stable. If the boiler then specifically asks for a reset, use only the reset procedure in the Baxi 800 Combi 2 user guide. Do not repeatedly reset the appliance if H.03.05 returns.
A reset cannot correct persistently low voltage. Repeated attempts may hide the pattern of the fault and delay the appropriate electrical or boiler checks.
Who should investigate H.03.05?
The right first contact depends on the evidence around the fault. If there is an acknowledged local power problem, contact the electricity network operator using its published fault service. If other equipment is behaving abnormally or there are signs of a property electrical fault, a qualified electrician may be needed. If the supply appears normal but the boiler continues to show H.03.05, arrange a boiler repair with a Gas Safe registered engineer.
You do not need to decide the cause before asking for help. Give the professional the exact code, model, time of occurrence and any recent power interruptions. They can determine which qualified trade or supply provider needs to take the next step.
What should you never do?
Do not remove the boiler casing, open a fused spur, alter a plug or socket, reset circuit breakers repeatedly, or inspect internal wiring. Do not use improvised leads, voltage converters, generators or inverters to try to run the boiler. Never bypass a safety device or ask an unqualified person to test the appliance.
If you notice burning smells, scorching, crackling, sparking or smoke, keep clear and switch off the electricity only if this can be done safely without approaching the affected equipment. Call the emergency services if there is immediate danger.
When should you book a boiler repair?
Arrange a repair when H.03.05 persists after the local supply is stable, returns repeatedly, or leaves you without dependable heating or hot water. The fact that the boiler starts again does not prove the issue is resolved.
When booking, say that the appliance is a Baxi 800 Combi 2 and quote H.03.05 exactly. Mention any power cut, flicker, generator use or work recently carried out on the property’s electrics. This is practical context, not a conclusion about the fault.
Gas and carbon-monoxide safety
H.03.05 is an electrical-supply code and does not itself indicate a gas leak. Normal gas-safety precautions still apply. If you smell gas, suspect a leak or a carbon-monoxide alarm sounds, stop using the boiler, move everyone into fresh air and call the National Gas Emergency Service on 0800 111 999 in Great Britain. Avoid electrical switches where gas may be present.
Headache, dizziness, nausea, breathlessness, collapse and loss of consciousness can be symptoms of carbon-monoxide exposure. Seek urgent medical help if anyone may have been affected.
Need the boiler repaired?
Your boiler fault is fixed for the agreed price, subject to eligibility and terms.
Quick answers
Does H.03.05 mean there is no electricity?
No. The boiler has detected voltage that is too low, and its display may still be working. A complete power cut is a different condition.
Does the code prove the boiler is faulty?
No. The code reports the low-voltage condition detected by the boiler, but it does not identify whether the source is external, within the property or within the appliance.
Can I test the voltage myself?
No. Mains electrical testing and internal boiler checks should be left to appropriately qualified professionals.
Is H.03.05 listed for the Baxi 800 Combi 2?
Yes. Baxi lists H.03.05 in its current Combi & System 2 group, which includes the Baxi 800 Combi 2.