Baxi H.03.05 fault codewhat it means and how to fix it
"Power supply voltage too low" on a Baxi Combi 2: the boiler pauses until the mains recovers and clears itself. Baxi's only remedy is to check the mains voltage; if it recurs, an electrician.
Take care
What H.03.05 means
H.03.05 is a temporary fault on the Combi 2 and System 2 defined in the manual as "power supply voltage too low" with the single remedy "check the mains voltage". The H means the boiler has stopped for now and will start again by itself: "disappears when the error condition is removed". Its relative, H.03.54, is "temporary flame loss — shutdown due to the power supply voltage being too low", the same sag catching the boiler mid-burn, and the manual's remedy for that runs through the electrode, gas supply and flue checks before ending "check the power supply voltage".
The board watches the mains it is fed because the gas valve, the fan and the ignition all need a supply within range; a brownout that would go unnoticed elsewhere in the house can drop the flame signal or leave a valve half-driven. Like E385 on the three-digit Baxis, this code points away from the boiler. A supply that sags at the boiler terminals is sagging because of a loose or high-resistance connection at the fused spur or the consumer unit, a circuit shared with a heavy appliance whose start-up pulls the voltage down, or an incoming supply that runs low at peak times, which other appliances will show too.
The board is the culprit only when a good supply is being misread, which an engineer establishes by measuring at the boiler terminals under load. No reset is needed or offered; the code goes when the voltage returns. The householder's useful observation is timing — H.03.05 that appears when the shower, oven or a tumble dryer starts is a wiring or loading problem, and that is an electrician's job before it is a heating engineer's.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Loose or high-resistance connection | At the fused spur, the boiler terminal strip or the consumer unit. Volts are dropped across the poor joint when the boiler draws current, and the board sees a low supply. | Common |
| Shared circuit with a heavy load | A boiler on a ring or spur that also feeds a shower or oven sees a dip each time the load starts. The code comes and goes with household activity. | Common |
| Incoming supply low | End-of-line or rural properties where the mains runs low at peak times. Lights dim and other appliances misbehave at the same moments. | Occasional |
| Board misreading a good supply | Concluded only after the supply has been measured at the boiler terminals under load and found normal. A PCB fault, and rare. | Rare |
What to try yourself
Notice when it happens. If H.03.05 appears as a shower, oven or dryer starts, or in the early evening, the supply is dipping under load and an electrician should check the boiler's circuit and connections.
Nothing to reset. The boiler restarts on its own when the voltage recovers. If it comes with H.03.54 the burner is losing its flame in the dips, and the same cause applies.
If it is frequent and nothing in the house explains it, book a heating engineer to measure the supply at the boiler; a good supply with a persistent code means the board.
Get a Gas Safe registered engineer to look at your Baxi H.03.05
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£400
Time on site
15 minutes
Nothing if the supply recovers. Electrical repair to a spur or connection £100–£250. PCB, if the supply is proven good, £250–£400.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Baxi / Potterton / Main main PCB
£150–£320Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Baxi fault codes
- E168Get it checkedTrade lists call E168 a Baxi PCB lockout, but no Baxi UK manual or error-code page prints it. Treat it as a board fault: one power cycle, then an engineer.Related: electronics and power
- E83Usually simple to fixE83 to E87: the boiler has lost contact with a connected control — a room unit or programmer wired to the board. Check the control first; the boiler itself is usually fine.Related: electronics and power
- E103Get it checkedSafety Assist codes E103, E104 and E107: the battery-backed safety board on the 600 and 800 is exhausted, faulty or out of step with the main board. Contact your installer; there is no reset.Related: electronics and power
- E.02.47Usually simple to fix"Connection to external device unsuccessful": a Baxi 200/400 Combi or Combi 2 has lost a control it was paired with. Check the control and its wiring; Baxi's remedy is the auto-detect function.Related: electronics and power
- E55Get it checked"Calibration Required": the gas valve has not been set up to the board after a PCB or valve change. An engineer's combustion-analyser job, not a fault a householder can clear.Related: electronics and power
- E317Get it checkedMains frequency out of range (shown on some boards as 162). The boiler, not the supply, is almost always the odd one out. Check other appliances, then call an engineer.Related: electronics and power
Own a Baxi boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.