Vokera E091 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera E091 fault code mean?
The E091 code appears on Vokera boilers — most notably the Synergy S — when the flue gas probe's internal hour counter surpasses 2,500 operating hours. The boiler uses this counter as a maintenance trigger to flag that the flue thermistor (the sensor that monitors exhaust gas temperature in the flue) has reached the end of its rated service interval and requires attention. You can confirm the current counter value by navigating to the INFO menu on the boiler and checking parameter I015. While E091 does not always mean the probe has physically failed, it does mean a Gas Safe engineer needs to inspect, clean, reset, or replace the probe before the boiler can be cleared. A related code, E090, signals that abnormally high flue gas temperatures were actually detected — if you see E090 rather than E091, the situation is more urgent. Code A09 is a further related alert covering general flue thermistor circuit issues.
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
- Hour counter threshold reached (maintenance trigger) Common
The most frequent reason for E091 is simply that the boiler has logged 2,500 hours of operation on the flue probe counter. This is a programmed service reminder built into the boiler's software rather than necessarily a sign of component failure. Once a Gas Safe engineer inspects and services the probe, the counter can be reset and normal operation resumed.
- Flue thermistor deterioration or failure Common
Extended exposure to hot exhaust gases gradually degrades the flue thermistor's accuracy. Carbon deposits build up on the probe surface, skewing its readings and causing the boiler to lock out. In this case the probe needs cleaning with a fine abrasive cloth or, if damaged beyond recovery, replacing outright.
- Loose, corroded, or damaged wiring to the flue probe Sometimes
Vibration and heat cycling over time can work connections loose or allow corrosion to develop at the probe's wiring terminals. A poor connection produces erratic resistance readings that the PCB may interpret as a fault even if the probe itself is in good condition.
- PCB misreading the probe signal Rare
In a minority of cases the printed circuit board receives incorrect data from the flue probe — not because the probe is at fault but because the PCB's own signal-processing circuitry has developed a fault. This requires specialist diagnostic equipment to confirm and is only investigated after the probe and wiring have been ruled out.
How to fix it
- Check your basic supplies and controls DIY safe
Before anything else, confirm that your gas supply is turned on, the boiler's power switch is on, and your room thermostat or programmer is calling for heat. If you use a prepayment meter, check you have credit. These quick checks rule out unrelated causes before assuming the boiler itself has a fault.
- Reset the boiler once or twice DIY safe
Vokera's own guidance advises waiting approximately two minutes before attempting a reset. On most Synergy S models, move the mode selector to the OFF/RESET position, pause, then return it to your desired mode (heating and hot water, or hot water only). If E091 clears and the boiler runs normally, it may return if the underlying cause is not addressed. Do not reset more than two or three times — persistent faults after repeated resets need engineer attention.
- Do not attempt to access the flue probe or INFO menu parameters yourself Gas Safe engineer
Navigating the engineer diagnostic menu (including parameter I015 for the hour counter) and handling the flue probe assembly are tasks for a Gas Safe registered engineer. Incorrectly resetting or mishandling the probe could mask a genuine safety issue with flue gas monitoring.
- Arrange a Gas Safe engineer visit for probe inspection and service Gas Safe engineer
The engineer will access the INFO menu to read the I015 hour counter value, remove the flue thermistor, inspect it for carbon build-up, and clean it using fine abrasive cloth if appropriate. If the probe shows signs of physical damage or gives readings outside its expected resistance range, it will be replaced. The counter is then reset so the boiler can resume normal operation.
- Engineer to check flue integrity and wiring Gas Safe engineer
While on site the engineer should visually inspect the flue terminal for blockages (leaves, debris, ice in cold weather) and confirm all sections of the flue system are correctly fitted and sealed. The probe's wiring harness and terminals should also be checked for corrosion or loose connections and replaced if necessary.
- PCB diagnosis if fault persists after probe work Gas Safe engineer
Should E091 return promptly after the probe has been cleaned or replaced and the counter reset, the engineer will test the PCB's ability to correctly interpret probe signals. PCB replacement is a last resort given the cost involved, so a reputable engineer will exhaust other possibilities first.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
E091 is not a fault a homeowner can resolve beyond the basic reset described above. A Gas Safe engineer is legally required for any work on the boiler's flue monitoring components. Contact a registered engineer directly or use Vokera's own fixed-price repair service for a transparent cost before work begins.
Parts you may need
- Flue gas thermistor / NTC probe (Vokera Synergy S) · from £35
- Probe wiring harness · from £52
- PCB (model-dependent, aftermarket) · from £160
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–£300, depending on the underlying cause.
Frequently asked questions
Is E091 dangerous — will my boiler keep running?
E091 is a maintenance-triggered warning rather than an emergency lockout, so it does not necessarily indicate an immediate safety risk. However, the flue gas probe is a key safety component — it monitors the temperature of combustion gases leaving the boiler. Ignoring the fault and continuing to run the boiler without having the probe inspected is not advisable, as a degraded or failed probe reduces the boiler's ability to detect potentially dangerous flue conditions. Book an engineer promptly rather than leaving it indefinitely.
Can the E091 hour counter be reset without replacing the probe?
Yes, in many cases. If the probe is inspected by a Gas Safe engineer and found to be in good working order — or after it has been cleaned of carbon deposits — the engineer can reset the I015 counter in the boiler's diagnostic menu without fitting a new part. The reset is a deliberate step in the service process, not a workaround, so as long as the probe itself passes inspection there is no need to replace it automatically.
What is the difference between E091, E090, and A09 on a Vokera boiler?
These three codes all relate to flue gas monitoring but indicate different conditions. E091 specifically means the flue probe's hour counter has exceeded 2,500 hours — it is primarily a service interval alert. E090 means the boiler actually detected abnormally high flue gas temperatures during operation, which is a more urgent condition suggesting a combustion or flue blockage issue. A09 is a broader flue thermistor circuit fault, covering wiring or sensor problems that prevent the PCB from receiving a valid probe signal. If you see E090 or A09, treat these as higher priority and arrange an engineer visit without delay.
Will an annual boiler service prevent E091 from appearing?
An annual service carried out by a Gas Safe engineer is the most reliable way to manage E091. During a service the engineer will check and clean the flue probe, inspect the wiring, and — if the hour counter is approaching or has exceeded 2,500 hours — reset it as part of the service record. Keeping to a yearly service schedule means the counter is unlikely to trigger an unexpected lockout, and any probe deterioration is caught early before it develops into a more costly repair.