Baxi E85 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E85 fault code mean?
The E85 code on a Baxi boiler signals a breakdown in communication between the boiler's internal components. The PCB (Printed Circuit Board) acts as the control hub of the boiler — every sensor, pump, valve, and ignition component sends and receives signals through it. When this communication chain breaks down, the boiler locks out as a precaution, preventing it from operating until the issue is identified and resolved. E85 sits within a wider family of Baxi communication fault codes (E83, E84, E85, E86, E87), each pointing to a slightly different communication path failing; E85 specifically indicates the PCB is not receiving the expected signals from one or more connected components. Because the fault is electrical in nature, it almost always requires a Gas Safe registered engineer to diagnose and repair.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Loose or damaged wiring connections Common
The most frequent trigger for E85 is a wiring connection that has worked loose, corroded, or become damaged inside the boiler. Each component — thermistors, fan, pump, gas valve — has a wired connection back to the PCB. If any of these becomes intermittent or breaks completely, the PCB interprets it as a communication failure and shuts the boiler down. Vibration over time, or a previous repair where connectors were not fully seated, can cause this.
- PCB fault or failure Common
If the PCB itself has developed a fault — through age, a power surge, moisture ingress, or a short circuit — it may no longer be able to send or receive signals correctly. This can present as a communication error even when the wiring and components themselves are perfectly fine. PCB failures can be gradual, producing intermittent faults before a full lockout.
- Faulty sensor or component sending incorrect signals Sometimes
Even if the PCB and wiring are intact, a malfunctioning component — such as a thermistor, flow sensor, or pressure sensor — can send garbled or out-of-range signals back to the PCB. The board cannot distinguish whether the signal is corrupt or genuine, so it raises a communication fault. In these cases, replacing the rogue component resolves the E85 without needing a new PCB.
- Software or firmware glitch Rare
On rare occasions, the boiler's control software can enter an inconsistent state — particularly after a power cut or voltage fluctuation. A one-off communication error of this type will often clear after a single boiler reset. If the fault returns immediately, a deeper hardware issue is almost certainly responsible.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on the boiler's control panel — on most Baxi models it is a dedicated button held for 3–5 seconds, or a symbol resembling a rotating arrow. Attempt a single reset. If the boiler fires up and runs normally, monitor it over the next day or two. If E85 returns, do not keep resetting; repeated resets can place additional stress on the PCB and may mask the underlying problem.
- Check for any obvious warning signs before calling an engineer DIY safe
Without opening the boiler casing, take note of anything unusual: a burning smell near the boiler, visible scorch marks on the casing exterior, flickering display lights, or any clicking or popping sounds. Also note whether any other fault codes appeared before E85 showed up. Write these observations down — they will help an engineer diagnose the fault more quickly and may reduce the time (and cost) spent on diagnostics.
- Check that the boiler's power supply is stable DIY safe
Confirm the boiler's fused spur switch is on and has not tripped. Check your consumer unit to make sure the circuit breaker for the boiler has not tripped. A voltage fluctuation or brief power interruption can occasionally trigger a one-off communication fault. If the electrics look fine and the reset did not clear the code, the fault lies inside the boiler.
- Contact a Gas Safe registered engineer to inspect the wiring and PCB Gas Safe engineer
An engineer will remove the boiler casing safely and carry out a systematic check of all wiring harnesses and connectors, looking for loose terminals, corrosion, or heat damage. They will test voltage levels at key points on the PCB and check whether individual components are sending correct signals. This diagnostic process pinpoints whether the fix is a simple reconnection job, a component swap, or a full PCB replacement — and ensures the repair is done safely and in compliance with Gas Safe requirements.
- Allow the engineer to repair or replace the faulty part Gas Safe engineer
If the fault is a loose or damaged wire, the engineer will resecure or replace the affected loom — a relatively quick and affordable fix. If a sensor or subsidiary component is sending bad signals, that part will be swapped out. If the PCB itself has failed, the engineer will source a Baxi-compatible replacement board. PCBs are model-specific and are often ordered directly from Baxi or an approved parts distributor, so there may be a short lead time before the repair can be completed.
Parts you may need
- Baxi PCB (model-specific) · from £280
- Wiring harness / loom · from £55
- NTC thermistor (flow or return sensor) · from £25
- Pressure or flow sensor · from £40
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
Is the E85 code dangerous?
E85 itself is not a gas-leak or carbon monoxide fault — it is an electrical communication error. However, because the boiler has locked out, you will have no heating or hot water until it is fixed. You should not attempt to open the boiler casing or interfere with any internal wiring. Call a Gas Safe registered engineer to carry out the repair safely.
What is the difference between Baxi fault codes E83, E84, E85, E86, and E87?
All five codes belong to the same family of internal communication faults on Baxi boilers. They each indicate that the PCB has lost communication with a specific component or data bus within the boiler — the precise component differs between codes. In practice, the diagnostic approach is similar for all of them: an engineer inspects the wiring connections and tests the PCB. If you see any code in the E83–E87 range, treat it the same way as E85 and arrange an engineer visit.
Could the E85 fault mean I need a new PCB, and how much does that cost?
Not necessarily — a significant proportion of E85 faults turn out to be loose wiring connections, which are much cheaper to fix (typically £120–£250 including labour). A full PCB replacement is required only when the board itself has failed. PCB parts for Baxi boilers generally cost £180–£400 depending on the model, and with labour the total can reach £450–£700. If a PCB replacement is quoted and your boiler is over 10 years old, it is worth comparing that cost against a new boiler installation, as modern A-rated boilers can reduce energy bills considerably.
My Baxi boiler is still under warranty — is the E85 fault covered?
Baxi boilers come with a standard 2-year warranty, extendable to up to 10 years when installed by a Baxi-accredited installer and serviced annually by a Gas Safe engineer. If your boiler is within the warranty period and has been serviced every year, an E85 communication fault is likely to be covered. Contact Baxi directly or check your warranty documents before arranging an independent engineer, as an unauthorised repair could affect your cover.