Viessmann EE Fault Code: Burner Lockout or External Fault Explained
The EE fault code on Viessmann boilers has two related interpretations that can overlap depending on your specific model. Most commonly — particularly on Vitodens series boilers — it signals that the burner has locked out because no flame was detected at start-up, or the flame signal was too weak to be considered safe. On other models and configurations, EE can indicate that the boiler's control board has picked up a fault from an externally connected device, such as a thermostat or sensor that is behaving unexpectedly. In both cases the boiler shuts down as a safety measure. Because the exact meaning varies by model, always cross-reference with your boiler's manual and treat this as a prompt to investigate rather than a single definitive diagnosis.
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.
What does the Viessmann EE fault code mean?
The EE fault code on Viessmann boilers has two related interpretations that can overlap depending on your specific model. Most commonly — particularly on Vitodens series boilers — it signals that the burner has locked out because no flame was detected at start-up, or the flame signal was too weak to be considered safe. On other models and configurations, EE can indicate that the boiler's control board has picked up a fault from an externally connected device, such as a thermostat or sensor that is behaving unexpectedly. In both cases the boiler shuts down as a safety measure. Because the exact meaning varies by model, always cross-reference with your boiler's manual and treat this as a prompt to investigate rather than a single definitive diagnosis.
Common causes
- Weak or absent flame signal at ignition Common
The boiler attempts to light the burner but either fails to produce a proper flame or the ionisation electrode cannot detect it reliably. This is the most common trigger for EE on Vitodens models. The electrode itself may be dirty, incorrectly positioned, or its connecting cable may be damaged, all of which prevent a clean flame signal reaching the control board.
- Air pressure switch fault Common
The air pressure switch confirms to the control board that the fan has reached the correct speed before ignition is allowed. If this switch is giving intermittent or incorrect signals — or if the fan has not reached a complete standstill between cycles — the board may lock out the burner and display EE. A faulty switch can also cause the boiler to cycle on and off repeatedly.
- Gas supply or gas valve problem Common
Insufficient gas pressure arriving at the boiler, a problem with the gas regulator, or a fault in the gas combination valve can all prevent the burner from establishing a stable flame at start-up. The boiler then locks out after its permitted number of ignition attempts.
- Faulty or incompatible external controls Sometimes
When EE is presenting as an external device fault, the most likely culprit is a wired thermostat, room sensor, or third-party control that is connected to the boiler but is sending signals outside the expected parameters. Loose wiring, a damaged connection, or a device that is incompatible with the boiler's control system can all produce this result.
- Flue gas recirculation or flue path restriction Sometimes
If combustion gases are being partially drawn back into the air intake — for example due to a poorly positioned flue terminal or a blockage — the burner may fail to light cleanly or the flame may be unstable. This can cause the boiler to lock out and display EE.
- Condensate drain blockage Rare
On condensing models, a blocked or restricted condensate drain can form part of the fault path that leads to an EE lockout, particularly in cold weather when the pipe may freeze. It is less often the sole cause but is included in Viessmann's official corrective-action checklist for this code.
- PCB fault Rare
A failing or damaged printed circuit board may misinterpret signals from the fan, ionisation electrode, or air pressure switch, causing a spurious EE lockout. PCB faults are generally only confirmed after all other components in the fault path have been checked and cleared.
How to fix it
- Check that your gas supply is working DIY safe
Turn on a gas hob or, if you have one, a gas fire. If they light normally, gas is reaching your home and a wider supply interruption is unlikely to be the cause. If you have no gas at all, contact your gas supplier before proceeding.
- Check your heating controls are calling for heat DIY safe
Make sure your room thermostat is set above the current room temperature, your programmer or timer is set to an 'on' period, and any TRVs on radiators are open. An incorrectly set timer is a surprisingly common reason a boiler appears to be at fault when it is actually behaving correctly.
- Check for a frozen condensate pipe DIY safe
In cold weather, the plastic condensate pipe that runs to an outside drain can freeze, causing the boiler to lock out. If you can see ice around an external section of white plastic pipe, you can carefully thaw it using warm (not boiling) water poured over the pipe or a warm cloth wrapped around it. Once thawed, reset the boiler and see whether it fires.
- Reset the boiler once or twice DIY safe
Locate the reset button on your Viessmann boiler — usually marked with a flame or reset symbol — and hold it for a few seconds as described in your user manual. If the boiler fires and runs normally, monitor it over the next few hours. If EE returns quickly, do not keep resetting: repeated lockouts on the same underlying fault should be investigated by an engineer.
- Check externally connected thermostats and wiring DIY safe
If you have a wired room thermostat or an external sensor connected to the boiler, check that the wiring is securely plugged in at both ends and that there are no obviously damaged cables. If you recently changed your thermostat or added a new control device, consider whether it is confirmed as compatible with your Viessmann model.
- Engineer to inspect the ionisation electrode and ignition components Gas Safe engineer
A Gas Safe engineer will remove and inspect the ionisation electrode for contamination or physical damage, check its gap setting, and test the connecting cable and ignition leads for continuity. A dirty or incorrectly set electrode is a common and relatively straightforward fix.
- Engineer to check gas pressure, gas train, and gas valve Gas Safe engineer
Using calibrated test equipment, the engineer will verify that gas is arriving at the correct working pressure and that the gas combination valve is operating within specification. Any fault in the gas train must be diagnosed and rectified by a competent Gas Safe registered engineer.
- Engineer to test the air pressure switch and fan Gas Safe engineer
The engineer will check that the fan is running correctly and reaching the right speed, inspect the pressure switch hose for blockages or splits, and test the switch itself. If the switch is giving intermittent or incorrect readings it will need to be replaced.
- Engineer to inspect the flue system Gas Safe engineer
The flue terminal and the entire flue run will be checked for blockages, leaks, and any signs that combustion gases could be recirculating back into the air intake. Any faults found in the flue system must be rectified before the boiler is returned to service.
- Engineer to check the condensate drain Gas Safe engineer
If a frozen or blocked condensate pipe is suspected and could not be resolved by the homeowner, the engineer will inspect the full condensate run, clear any blockage, and check that the pipe is correctly routed and insulated to prevent future freezing.
- Engineer to evaluate the PCB if all other checks are clear Gas Safe engineer
If every other component in the fault path has been checked and found to be in good order, the engineer may suspect the PCB. This is diagnosed by process of elimination and the engineer should be confident in their diagnosis before recommending replacement, as PCBs are expensive. The overall age and condition of the boiler should also be considered at this stage.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the EE code returns after your own checks, or if you are unsure at any point, contact a Gas Safe registered engineer. Ask for someone with Viessmann experience if possible. You can verify any engineer's registration at gassaferegister.co.uk.
Parts you may need
- Ionisation / flame detection electrode · from £35
- Ignition lead and electrode cable set · from £45
- Air pressure switch · from £75
- Gas combination valve · from £180
- Fan assembly · from £150
- PCB (control board) · from £250
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 £150–£420, depending on the underlying cause.
Frequently asked questions
Can I reset my Viessmann boiler myself when EE is showing?
Yes, a single reset is a reasonable first step and Viessmann acknowledges that some fault codes will clear this way. Press and hold the reset button as described in your user manual. If the boiler fires up and runs normally, monitor it carefully. However, if EE returns within a short time, stop resetting and call an engineer — repeated lockouts on an underlying fault will not fix themselves and may mask a worsening problem.
How much does it cost to fix a Viessmann EE fault in the UK?
Most EE repairs fall in the range of £150 to £420 including parts and labour. Wiring and connection issues are at the lower end, while air pressure switch replacement typically costs around £200 and a gas valve repair or replacement can reach £350 to £420. If the PCB turns out to be the cause, expect to pay £400 to £500 — and at that point it is worth asking your engineer whether a newer boiler would be a better investment, particularly if the boiler is over 10 years old.
Why does my Viessmann boiler show EE and then fire normally for a while before locking out again?
Intermittent EE faults that clear temporarily after a reset are a classic sign of an air pressure switch that is beginning to fail. The switch works correctly when conditions are ideal but gives unreliable signals as it deteriorates, causing the boiler to lock out sporadically. A dirty ionisation electrode can behave similarly — producing a flame signal that is just strong enough sometimes but drops below the detection threshold under slightly different conditions. Either way, an engineer needs to run a proper diagnostic rather than relying on resets.
Does EE always mean a burner fault on Viessmann boilers?
Not always. On most Vitodens models EE points to a burner lockout caused by a missing or weak flame signal, but on some Viessmann models it indicates a fault with an externally connected control device such as a thermostat or sensor. Viessmann themselves advise that the correct diagnosis can vary between models, which is why cross-referencing your specific model's documentation — and ideally consulting a Viessmann-trained engineer — is important before assuming any single cause.