The Baxi H.03.05 code means “Power supply voltage too low.” The boiler control has detected that its electrical supply is below the level it expects for normal operation.
This message does not tell you why the voltage is low, and it does not prove that the boiler’s circuit board has failed. The issue may be temporary or may need attention to the appliance supply or the property’s electrical installation. Electrical testing is not a homeowner task.
Which Baxi boilers show H.03.05?
Baxi’s current error-code table lists H.03.05 for the Baxi 200, 400, 600 and 800 Combi 2; Baxi Assure 500 Combi 2; Baxi 600 and 800 System 2; and Baxi Assure 500 System 2.
Confirm the complete model name from the boiler front or your paperwork. Do not remove a casing panel. The exact model matters when checking the correct user guide and reset control.
What does low power-supply voltage mean?
A modern boiler relies on a stable mains supply for its controls and components. H.03.05 means the boiler has detected supply voltage below its expected operating level. It may pause or limit operation rather than continue under an unsuitable electrical condition.
The code does not identify where the supply problem is. It could coincide with a wider local supply disturbance, an issue affecting the circuit serving the boiler, or a condition within the appliance. Only appropriate professional checks can separate those possibilities.
Is H.03.05 a gas fault or an electrical fault?
The manufacturer’s stated meaning is electrical: the power-supply voltage is too low. However, the boiler remains a gas appliance, and internal access may involve both electrical and gas-related components. Keep the casing closed and use a suitably qualified professional.
Do not test voltage at the boiler, its fused connection, a socket, the consumer unit or internal terminals. Do not replace a fuse, alter wiring, use an extension lead or move the boiler to another supply as an experiment.
Safe checks you can make
- Write down H.03.05 exactly as shown.
- Confirm the complete boiler model.
- Note whether heating, hot water, or both have stopped.
- Check whether lights or other appliances in the home are behaving unusually.
- Ask whether nearby properties have also lost power, if this can be done safely.
- Record whether the code followed a power cut, flicker or other electrical work.
- Look only at the visible position of the relevant consumer-unit switch; do not remove covers or repeatedly reset a tripping circuit.
If there is a known local power interruption, wait for the normal supply to return. Do not run the boiler from a generator, extension lead, voltage device or temporary connection unless the boiler manufacturer and a competent electrician have specifically approved the arrangement.
What if a circuit breaker has tripped?
A tripped breaker may indicate a fault that needs investigation. If the circuit will not remain on, if it trips again, or if you are unsure which control relates to the boiler, leave it off and contact a competent electrician or heating professional.
Do not repeatedly operate a breaker, remove a fuse carrier, open the consumer unit or attempt wiring work. Electrical Safety First advises using a registered competent electrician for electrical work rather than attempting uncertain DIY.
Can I reset the boiler?
If the electricity supply appears normal and the exact Baxi user guide permits it, one reset using the normal boiler control is reasonable. Baxi reset methods vary, so use the correct guide.
If H.03.05 returns, do not keep resetting the boiler. Repeated resets will not correct a persistent low-voltage condition. Note whether the code returns immediately or when the boiler tries to start, then arrange professional help.
When is the situation urgent?
Switch off the affected circuit only if it is safe to use the normal labelled control, and seek urgent electrical help if you notice burning smells, scorching, buzzing, crackling, heat at a switch or socket, sparks, smoke or repeated tripping. Do not touch damaged equipment.
If there is smoke or fire, leave the property and call 999. If you smell gas or a carbon-monoxide alarm sounds, leave the property, avoid electrical switches and flames, and call the National Gas Emergency Service on 0800 111 999 from a safe place.
What should you tell the professional?
Provide the full H.03.05 code, boiler model, what stopped working, whether other electrics were affected and whether the message followed a power interruption or electrical work. A display photograph can help if it can be taken without touching or opening the appliance.
For more homeowner-safe code explanations, visit the boiler help hub.
Is Baxi H.03.05 preventing normal boiler operation?
Book a fixed-price boiler repair aimed at fixing the fault, with the booking price clear before the visit.
Quick answers about Baxi H.03.05
What does Baxi H.03.05 mean?
It means the boiler has detected that its power-supply voltage is too low.
Does H.03.05 mean the boiler PCB is faulty?
No. The code does not identify one failed part or say where the low-voltage condition originates.
Can a power cut cause H.03.05?
The code may coincide with a wider supply disturbance, but the display alone cannot establish the cause.
Can I test the voltage myself?
No. Do not use a meter or open the boiler, fused connection or consumer unit. Electrical checks belong to a competent professional.
Should I keep resetting the boiler?
No. Use one reset only if the exact guide permits it and the supply appears normal. If the code returns, arrange professional help.