Vokera E086 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera E086 fault code mean?
The E086 fault code on Vokèra Vision Plus series boilers signals an abnormal temperature reading linked to the flow or return circuit — most commonly interpreted as a flow temperature sensor fault, an over-temperature differential alarm, or a return-line temperature anomaly. In plain terms, the boiler's control system has detected that the temperature difference between the flow and return water is outside safe operating limits, or that a thermistor is reporting readings that do not make sense. The boiler locks out as a protective measure to prevent damage from overheating or undetected circulation problems. Because E086 sits in the Vision Plus fault table alongside E084 (flow sensor over-temperature or differential alarm) and E090 (flue gas over-temperature probe fault), the underlying issue nearly always involves either the NTC thermistor, water circulation, or a combination of both. Vokèra's own manual advises that all reset faults — E086 included — require an engineer unless specifically stated otherwise.
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
- Faulty or degraded NTC thermistor Common
The flow or return NTC temperature sensor is the most frequent culprit. As thermistors age or suffer heat stress, their resistance characteristics drift, causing the boiler's PCB to receive inaccurate temperature data. The boiler then triggers E086 as a precaution. Confirming this requires a resistance check with a multimeter — an engineer's task.
- Restricted water circulation or blocked pump Common
If hot water cannot circulate freely, the temperature difference between flow and return rises sharply, tripping the differential alarm. A sluggish or seized central heating pump — often caused by heating sludge, airlocks, or lack of a magnetic filter — is a typical underlying cause when the thermistor itself tests within spec.
- Limescale or sludge buildup in the heat exchanger Common
Hard water areas are particularly prone to limescale deposits inside the primary heat exchanger. Sludge accumulation has a similar effect. Both restrict flow, cause localised overheating, and produce erratic temperature readings that trigger E086. A power flush or chemical clean is usually needed to resolve this.
- Loose or corroded wiring to the thermistor Sometimes
Vibration from the pump and normal thermal cycling can loosen the connectors on the thermistor wiring harness over time. A poor connection causes intermittent or wildly incorrect temperature signals to the PCB, producing the same fault even when the sensor itself is undamaged.
- Flow and return pipes incorrectly connected Sometimes
If the boiler was installed or recommissioned with the flow and return connections reversed, the return sensor will consistently read temperatures it was not designed to see, triggering an immediate differential alarm. This is more likely to surface after a recent installation or system alteration.
- PCB fault Rare
In a minority of cases the PCB itself misinterprets signals from a perfectly healthy thermistor, or its onboard logic fails. This is typically only confirmed after a thermistor and wiring check have both come back clear.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on the Vokèra Vision Plus control panel — it is usually marked with an 'R' or a flame/lock symbol. Press and hold it for 3–5 seconds until the fault indicator stops flashing, then release and allow the boiler to restart. If E086 clears and the boiler runs normally, monitor it closely over the next hour. If the code returns promptly, do not keep resetting repeatedly, as this can mask a worsening fault.
- Check system pressure DIY safe
Look at the boiler's pressure gauge. For most Vokèra boilers when cold, the correct operating pressure is between 1.0 and 1.5 bar. If it reads below 1.0 bar, top up slowly using the filling loop beneath or beside the boiler until the gauge sits around 1.2–1.5 bar, then attempt a reset. Never pressurise beyond 2.0 bar.
- Bleed radiators if the system feels sluggish DIY safe
Trapped air reduces circulation, which can cause temperature differentials to spike. Using a radiator bleed key, open each radiator bleed valve until water (not air) flows out, then close it. Start with the radiator furthest from the boiler. Re-check system pressure afterwards and top up if necessary.
- Check that your gas supply is on DIY safe
Ensure the gas isolation valve on the supply pipe to the boiler is fully open (handle inline with the pipe). Confirm other gas appliances in your property — such as a hob or gas fire — are working normally. If there is no gas supply, contact your gas supplier rather than an engineer.
- Arrange a Gas Safe engineer to test the thermistor Gas Safe engineer
A registered engineer will use a multimeter to measure the thermistor's resistance at a known water temperature and compare it against the manufacturer's specification curve. A reading outside tolerance confirms the sensor has failed and needs replacing. The boiler casing must be removed for this check, which legally requires Gas Safe registration.
- Have the engineer inspect wiring and connections Gas Safe engineer
If the thermistor tests within specification, the engineer will examine the wiring harness, connectors, and PCB terminals for corrosion, heat damage, or loose pins. A damaged loom or connector can be repaired or replaced at relatively low cost compared with component replacement.
- Have the engineer assess circulation and pump condition Gas Safe engineer
If wiring and the thermistor are both healthy, the engineer will check pump output, look for signs of heating sludge, and assess whether a power flush is needed. A blocked or failing pump will need to be serviced or replaced. A magnetic filter installation at the same visit will help prevent recurrence.
- Call a Gas Safe registered engineer if E086 persists or returns Gas Safe engineer
If the code reappears after your basic checks, book a Gas Safe registered engineer. You can verify any engineer's credentials at gassaferegister.co.uk. Continuing to run or repeatedly reset a locked-out boiler risks damaging the heat exchanger and other internal components, and the Vokèra manual explicitly advises professional attention for this fault.
Parts you may need
- NTC thermistor (flow or return sensor) · from £20
- Central heating pump (compatible replacement) · from £80
- Wiring harness / thermistor connector · from £15
- PCB (Vision Plus series) · from £180
- Magnetic system filter (e.g. Fernox TF1 or equivalent) · from £55
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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I fix a Vokèra E086 fault myself?
Very little of this repair is DIY-safe. You can safely try resetting the boiler once or twice, check and top up system pressure, and bleed radiators — but any internal inspection or component work requires the boiler casing to be removed, which legally must be done by a Gas Safe registered engineer. Attempting internal repairs without the correct registration is both dangerous and illegal in the UK.
How much does it typically cost to fix a Vokèra E086 fault?
For the most common cause — a failed NTC thermistor — expect to pay roughly £100–£150 all-in, including parts (around £20) and an hour or two of labour. If the pump needs replacing, budgets of £200–£350 are more realistic. A power flush to clear sludge typically runs £300–£400. In rare cases where the PCB is at fault, costs can rise to £400–£550 or more, at which point it is worth comparing that against the cost of a new boiler, particularly if the Vokèra is older.
Why does my Vokèra Vision Plus keep showing E086 after I reset it?
A fault that returns quickly after resetting usually means the underlying problem — a degraded thermistor, poor circulation, or sludge buildup — has not been resolved. Repeatedly resetting without fixing the root cause can stress other components. Book a Gas Safe engineer to diagnose the fault properly rather than continuing to reset.
How can I prevent the E086 fault from coming back?
The most effective preventive measures are: fitting a magnetic system filter (such as a Fernox TF1 or Magnaclean) to capture iron oxide sludge before it reaches the boiler; dosing the central heating system with a quality inhibitor fluid; having an annual boiler service — which costs roughly £90–£140 — so a Gas Safe engineer can inspect sensors, connections, and pump condition before they deteriorate; and addressing any hard-water limescale issues with appropriate water treatment if you live in a hard-water area.