Vokera E090 Fault Code: Causes, Fixes & Repair Costs
What does the Vokera E090 fault code mean?
The E090 fault code appears when your Vokera boiler detects a problem related to the flue gas over-temperature probe. This sensor continuously monitors the temperature of exhaust gases as they leave the boiler through the flue. When E090 is triggered, it means either the sensor has recorded genuinely high flue gas temperatures — pointing to a combustion or heat transfer problem — or the probe itself has deteriorated and is sending inaccurate readings to the PCB. Either way, the boiler locks out as a precaution. Because this fault directly involves flue gas safety and the risk of overheating, it should be treated seriously. A single reset attempt is reasonable, but if the code returns, a Gas Safe registered engineer must be called.
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 flue over-temperature probe or thermistor Common
The probe itself is the most frequent culprit. Over time the thermistor can degrade, develop a drift in its resistance, or fail outright, causing it to report temperatures that are either falsely high or erratic. The PCB interprets this as a safety event and triggers E090 even when flue temperatures are perfectly normal.
- Dirty or scaled primary heat exchanger Common
Limescale build-up or soot deposits on the heat exchanger reduce its ability to transfer heat into the water. This forces exhaust gases to exit at a higher temperature than designed, which the over-temperature probe correctly detects and flags. Regular servicing is the best way to prevent this.
- Blocked or partially restricted flue Sometimes
Debris such as leaves, bird nests, or even ice and snow can partially obstruct the flue terminal. A restriction causes exhaust gases to back up and heat up inside the flue, leading the probe to trip. You can visually inspect the external flue terminal yourself, but any internal inspection or clearance must be done by an engineer.
- Wiring fault or poor connection to the probe Sometimes
A corroded connector, chafed wire, or loose terminal between the flue probe and the PCB can produce an open or short circuit that mimics a high-temperature reading. The boiler cannot distinguish this from a genuine over-temperature event and locks out accordingly.
- PCB fault Rare
In rare cases the PCB itself misinterprets the signal from an otherwise healthy probe. This is usually only confirmed after the probe and wiring have both been checked and found to be in good order.
How to fix it
- Check for a working carbon monoxide alarm DIY safe
Before doing anything else, confirm your CO alarm is present, powered, and functioning. The E090 code involves flue gases, and any fault in this area carries a small but real risk of CO exposure. If your alarm is sounding, evacuate immediately and call the Gas Emergency Helpline on 0800 111 999.
- Visually inspect the external flue terminal DIY safe
Go outside and look at where the flue exits the building. Check for obvious blockages such as leaves, bird nesting material, ice, or damage to the terminal grille. Do not attempt to dismantle the flue — this visual check only. If you spot a clear external obstruction you can safely remove (e.g. loose debris at the terminal), do so carefully.
- Attempt a single boiler reset DIY safe
On most Vokera models, press and hold the reset button (marked R) for three to five seconds until the fault code stops flashing, then release. On dial-type models, turn the selector to the reset position and hold briefly. Allow the boiler to restart and observe whether E090 returns. Attempt this no more than two or three times in total — repeated resets on a fault involving flue gas temperatures are not safe and will not fix an underlying problem.
- If the fault clears, monitor the boiler closely DIY safe
If the reset works and the boiler runs normally, keep an eye on it over the next few hours. Book a service with a Gas Safe engineer soon — a fault that appeared once and cleared may be an early warning of a deteriorating probe or a build-up of scale that will trigger E090 again.
- If E090 returns, stop resetting and call a Gas Safe registered engineer Gas Safe engineer
A recurring E090 code indicates a genuine fault that needs professional diagnosis. The engineer will use test equipment to check the flue over-temperature probe's resistance values, inspect and clean the primary heat exchanger if scaled, examine the flue system for restrictions, and test the wiring harness and PCB connections. Do not continue to reset the boiler repeatedly — doing so risks damage to the heat exchanger and, in a worst case, a CO-related safety incident.
Parts you may need
- Flue over-temperature thermistor / probe · from £25
- Thermistor wiring harness and connectors · from £20
- Primary heat exchanger (if replacement required) · from £180
- PCB (printed circuit board) · from £220
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–£320, depending on the underlying cause.
Frequently asked questions
Is the Vokera E090 code dangerous?
It can indicate a genuine safety issue because it relates to flue gas temperatures — the same system that, if faulty, could contribute to carbon monoxide risks. You should not ignore it or keep resetting the boiler without investigating the cause. Make sure you have a working CO alarm fitted and call a Gas Safe engineer if the fault returns after a reset.
Can I reset a Vokera boiler showing E090 myself?
Yes, one or two resets are reasonable as a first step, and occasionally a one-off sensor glitch will clear this way. However, if E090 keeps coming back, resetting is not a fix — it simply restarts a boiler that has locked out for a safety reason. Stop resetting and get a Gas Safe engineer to diagnose the root cause.
How much does it cost to fix a Vokera E090 fault in the UK?
Most people with this fault pay somewhere between £100 and £320 all in, depending on what is found. A straightforward thermistor replacement including parts and labour typically falls in the £100–£200 range. A heat exchanger clean or power flush tends to cost £150–£300. If the PCB is at fault that can push costs higher — potentially £250–£450 or more — but PCB failure is an uncommon cause of E090. Getting a few quotes from Gas Safe engineers is always worthwhile.
Could a dirty heat exchanger cause E090 on my Vokera boiler?
Yes, this is one of the more common causes. When limescale or soot coats the heat exchanger, it cannot transfer heat efficiently into the water circuit. The exhaust gases consequently leave the boiler at a higher temperature than normal, which the flue over-temperature probe correctly detects and flags. An annual boiler service that includes a heat exchanger check is the best way to avoid this scenario.