Baxi E83 fault codewhat it means and how to fix it
E83 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.
Before you start
What E83 means
Baxi print E83 through E87 as a single line — "Communication error" in the 600, 800, Compact and Duo-tec 2 tables and "E83 / E84 / E85 / E86 / E87 — No communication" on the professional page — and they mean the same thing: the board expected to hear from a device on its communication bus and did not.
On these boilers that device is a Baxi room controller or programmer wired to the two-wire bus terminals rather than to a simple switched-live input, and the five numbers distinguish which channel or which stage of the handshake failed, a detail the householder does not need because the remedy is the same for all of them. The boiler does not lock out for a communication error in the way it does for a flame or fan fault; it shows the code and, depending on the model and what it has lost, either carries on with its own settings or stops responding to demand until the control is back. Baxi give no reset for it. Neither the 600 user guide's reset list nor the installation manual's "possible to relight" list includes any of 83 to 87, and since the fault is a missing conversation rather than a stored lockout, the code clears itself when the control is found again.
The practical order is: the control's own display and batteries, if it has them; the two wires between control and boiler, especially where they enter the boiler's terminal strip; then the board. A boiler that was working with an old on/off thermostat and shows E83 after a new smart thermostat was fitted has almost always had the new control wired to the bus terminals when it should have been on the switched-live pair, or the other way round.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Control disconnected or unpowered | A wired room unit with a flat battery, a programmer unplugged from its backplate, or a control removed for decorating. The boiler reports no communication because there is nobody to talk to. | Common |
| Bus wiring | The two-core cable between the control and the boiler's bus terminals broken, loose at the terminal strip, or reversed on a control that cares about polarity. | Common |
| Wrong terminals after a new thermostat | A third-party thermostat wired to the bus terminals, or a Baxi bus control wired to the switched-live terminals. The code appears the day the control is fitted. | Occasional |
| Board communication circuit | The last suspect. The bus interface on the PCB has failed; ruled out by testing with a known-good control at the boiler terminals. | Rare |
What to try yourself
Look at the room control. If it has a blank display or dead batteries, replace them; if it has been unplugged from a wall plate, refit it.
If the code appeared right after a new thermostat was fitted, tell whoever fitted it — it is very likely on the wrong pair of terminals. Do not open the boiler's wiring cover yourself.
Baxi give no reset for E83–E87 and none is needed; the code clears when the control is back in contact. If it stays, book an engineer to check the bus wiring at the boiler.
Get a Gas Safe registered engineer to look at your Baxi E83
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–£150
Time on site
15 minutes
Batteries or refitting a control cost nothing. Rewiring a control £80–£150. PCB, rarely, £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
Own this appliance and had this code? . Nothing is published until a person has read it.
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
- 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
- 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 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.