Warmflow E35 Fault Code: Causes, Fixes & Repair Costs
What does the Warmflow E35 fault code mean?
The E35 fault code on a Warmflow boiler indicates that the control board has lost confidence in the ionisation (flame-sensing) electrode. During every ignition attempt, the PCB passes a tiny electrical current through the flame itself — if a signal returns, the boiler knows combustion is happening safely and continues to supply gas. If that signal fails to come back within a few seconds of the spark firing, the boiler cuts the gas supply, locks out, and displays E35 as a safety precaution. This prevents unburned gas from building up inside or around the appliance. The root cause is most often a dirty or worn electrode, but wiring faults, combustion issues, or a failing PCB can also trigger this code.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Dirty or contaminated ionisation electrode Common
The flame-sensing rod sits directly in the path of the burner flame and gradually accumulates a dark, carbonised or scaly residue — particularly in hard water areas. Once the coating is thick enough, it insulates the rod and prevents it from reading the flame current, causing a lockout even when the burner has actually lit successfully. This is by far the most common trigger for E35 and the first thing a Gas Safe engineer will inspect.
- Worn or degraded electrode Common
Constant exposure to high temperatures and the electrical stress of repeated ignition cycles causes the sensing rod to erode over time. A worn electrode may still look visually acceptable but can no longer generate a reliable signal. Boilers that haven't been serviced in two or more years are particularly prone to this.
- Loose or corroded wiring connections Sometimes
The lead running from the ionisation electrode back to the PCB can develop corrosion at its terminals, or the plug connector can work loose due to vibration. Even a marginal increase in resistance on this low-current signal circuit is enough to fool the control board into thinking the flame isn't present.
- Combustion or burner problem Sometimes
If the burner itself is partially blocked, the air-to-gas mixture is off, or gas pressure to the appliance is low, ignition may be weak or inconsistent. A feeble flame produces a weaker ionisation signal, which the PCB may not register as sufficient proof of combustion, triggering E35.
- Faulty PCB Rare
The PCB generates the reference voltage used to measure the ionisation current and interprets the returning signal. If the relevant circuit on the board has failed, it may misread a perfectly good electrode as faulty. This is a less common cause but should be considered once the electrode, wiring, and combustion have all been checked and found satisfactory.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (or follow the reset procedure in your Warmflow user manual) for the number of seconds specified. Allow the boiler to complete a full ignition sequence. If it fires up and runs normally, monitor it over the next day or two. A one-off lockout can occasionally occur without a persistent fault. Do not reset more than two or three times in total — repeated resets without a successful start will not fix the underlying problem and may mask a developing fault.
- Check your gas supply DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, your gas supply may be interrupted — contact your gas supplier rather than an engineer. If other appliances are fine, the issue is specific to the boiler.
- Check boiler system pressure DIY safe
Look at the pressure gauge on the boiler fascia. Warmflow boilers typically require between 1.0 and 1.5 bar when cold. If pressure has dropped below 0.5 bar, top it up via the filling loop to around 1.2 bar, then attempt another reset. Low pressure alone is unlikely to cause E35, but it rules out a secondary issue.
- Contact a Gas Safe registered engineer to inspect and clean the ionisation electrode Gas Safe engineer
An engineer will isolate the boiler, remove the burner access panel, and extract the ionisation electrode. They will clean it carefully — typically with fine wire wool or a specialist electrode cleaner — check the sensing gap, and refit it. This is usually a straightforward 20–30 minute job and resolves E35 in the majority of cases. Do not attempt to access burner components yourself; this work requires Gas Safe registration under UK law.
- Engineer to inspect electrode wiring and connections Gas Safe engineer
If cleaning the electrode does not clear the fault, the engineer will trace the signal lead back to its connector on the PCB, checking for corrosion, heat damage, or loose terminals. A faulty lead is inexpensive and quick to replace.
- Engineer to replace the ionisation electrode if worn or damaged Gas Safe engineer
Where cleaning is insufficient because the electrode is physically degraded, the engineer will fit a new one. Genuine Warmflow replacement electrodes typically cost £20–£60 for the part, and the job is usually completed within a single visit.
- Engineer to assess combustion quality and gas pressure at the appliance Gas Safe engineer
If the electrode and wiring are both sound, the engineer will use a flue gas analyser to check combustion efficiency and verify that gas inlet pressure at the boiler meets Warmflow's specification. A poorly balanced air-to-gas ratio or low gas pressure can produce a weak flame that fails ionisation checks even with a healthy electrode.
- Engineer to assess or replace the PCB if all other causes have been ruled out Gas Safe engineer
A PCB replacement is a last resort once the electrode, wiring, and combustion have all been confirmed as satisfactory. The engineer may be able to test the ionisation circuit on the board in isolation. PCB replacements are significantly more expensive, so a thorough diagnosis beforehand is important to avoid unnecessary cost.
Parts you may need
- Ionisation / flame-sensing electrode · from £35
- Electrode signal lead and connector · from £20
- Burner gasket / sealing kit · from £15
- PCB (control board) · from £280
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 £90–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Warmflow E35 myself?
The initial reset is something you can safely try yourself, and you should also confirm your gas supply is working and that boiler pressure is within the normal 1.0–1.5 bar range. Beyond that, the fault involves components inside the boiler's combustion chamber — the ionisation electrode, burner, and associated wiring — which must only be accessed by a Gas Safe registered engineer. Attempting this work without registration is illegal in the UK and carries serious safety risks.
How much does it cost to fix E35 on a Warmflow boiler?
For the most common repair — an engineer visit to clean or replace the ionisation electrode — expect to pay roughly £90 to £220 all in, depending on your location and whether the electrode needs replacing or just cleaning. If the fault turns out to be a wiring issue the cost is usually towards the lower end of that range. A PCB replacement, which is needed only in a minority of cases, can cost £350 to £500 or more and is best discussed with your engineer before proceeding, especially on an older boiler where a replacement might be better value.
Why does E35 keep coming back on my Warmflow boiler?
A recurring E35 code after the electrode has been cleaned usually points to one of three things: the electrode is worn beyond the point where cleaning helps and needs replacing; there is an intermittent wiring or connector fault that wasn't fully resolved the first time; or there is an ongoing combustion issue (low gas pressure or incorrect air-to-gas mixture) producing a weak flame that the electrode struggles to detect reliably. Ask your engineer to carry out a full combustion analysis and inspect the signal lead as well as the electrode itself.
Does annual servicing prevent the E35 fault?
In most cases, yes. Electrode cleaning is a standard part of a Warmflow annual service, so a boiler that is serviced every year is far less likely to develop the carbon or scale build-up that causes E35. Warmflow also requires annual servicing as a condition of the boiler's warranty — skipping a service can invalidate your cover, so it is worth booking one regardless.