Baxi E103 fault codewhat it means and how to fix it
Safety 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.
Take care
Get a Gas Safe registered engineer to look at your Baxi E103
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 E103 means
The 600 and 800 Combi and System carry a second circuit board Baxi call Safety Assist, with its own battery, and three codes belong to it. Baxi's professional page: "E103 — Safety Assist battery is exhausted", "E104 — Faulty Safety Assist PCB", "E107 — PCBs not synchronised". The 600 Combi user guide does not explain them but singles them out in its reset instructions: "For 103, 104, 107 & 'Bat' please contact your installer, annual service provider or Baxi Customer Support", and "Bat" is the display's low-battery warning that precedes E103.
The point of the second board is to keep the boiler's safety monitoring alive through a mains interruption, so a boiler whose Safety Assist battery is flat is a boiler Baxi will not let run on the main board alone. None of the three is in the installation manual's list of codes that relight from the Reset button, and the battery is not a user-replaceable cell in the way a smoke alarm's is; it is changed by an engineer with the case off. E104 and E107 are board faults — the Safety Assist PCB itself, or the link between it and the main board — and are diagnosed by substitution.
For the householder the three codes come to the same instruction Baxi give, and the only distinction worth making is urgency: E103 after "Bat" has been showing for a while is a planned battery change, while E104 or E107 appearing from nowhere is a board that needs looking at before the boiler is trusted to run unattended. These codes belong to the 600 and 800 only; the Duo-tec 2, Compact, Assure and selector-switch Baxis have no Safety Assist board.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Safety Assist battery exhausted (E103) | The backup cell on the Safety Assist board has run down, usually after the display has shown "Bat" for some time. Changed by an engineer with the case off. | Common |
| Safety Assist PCB fault (E104) | The secondary board has failed a self-test. Replaced as a unit; Baxi do not publish a repair. | Occasional |
| Boards not synchronised (E107) | The main PCB and the Safety Assist board disagree, typically after one has been replaced without the pairing procedure, or when the link between them is disturbed. | Occasional |
| Connection between the boards | A ribbon or plug between the two PCBs loose or corroded gives E107 on two good boards. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
30 minutes
Battery change within a service visit, £80–£150. Safety Assist PCB £150–£300 fitted.
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
- 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
- E385Get it checkedUnder-voltage (shown on some boards as 165): the supply at the board dropped below what it will run on. Usually a house-wiring or supply problem the boiler is reporting, not causing.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 support portal
- Manufacturer technical and service documentation
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.