Buderus 4E 278 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 4E 278 fault code mean?
The 4E fault code, paired with operating code 278, means the boiler's control board (the UBA) has failed a sensor test — specifically, it cannot properly read or recognise the combustion chamber temperature sensor. Buderus groups its fault codes into families: the '4' family covers temperature-sensing problems, and 4E / 278 indicates a sensor test failure within that family. Depending on how long the fault persists, the boiler may enter a temporary blocking state (it shuts down but can recover automatically if the condition clears) or a full lockout (requiring a manual reset, shown by a flashing 'H' symbol on the display alongside the fault code). If you see a flashing 'H', a reset is needed before the boiler will fire again.
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 NTC temperature sensor Common
The combustion chamber NTC sensor is the most common culprit. Over time these sensors drift out of range, develop internal faults, or fail outright. When the UBA control board reads a resistance value it cannot make sense of, it logs code 278 and shuts the boiler down. Sensor failure is the single most frequent cause of this fault.
- Damaged or short-circuited sensor wiring Common
The cable connecting the combustion chamber sensor to the UBA board can develop a short circuit — sometimes due to physical damage, chafing against the boiler casing, or heat exposure. A shorted wire produces an incorrect signal at the board, triggering the 4E fault even if the sensor itself is fine.
- Open circuit in the sensor cable Sometimes
The opposite of a short: a break somewhere in the sensor wiring means no signal reaches the UBA at all. If this open-circuit condition persists, the boiler typically escalates from a blocking fault to a full locking fault. The engineer will need to test continuity along the full length of the cable.
- Loose or corroded UBA control board connector Sometimes
The UBA module (located behind the cover at the lower left of the boiler) connects to sensors via a multi-pin plug. Bent pins, corrosion, or a connector that has worked loose can mimic a sensor failure, producing the 4E / 278 code without the sensor itself being defective.
- Limescale build-up causing overheating Sometimes
In hard-water areas, scale deposits inside the heat exchanger restrict water flow and can cause localised overheating. This places the temperature sensor under thermal stress, producing erratic readings that the UBA interprets as a sensor fault. Descaling or a system flush may be needed alongside sensor replacement.
- Failed PCB / UBA control board Rare
If the sensor, wiring, and all connectors test correctly, the UBA board itself may be at fault — for example due to component damage, condensation ingress, or a failed track on the board. This is a less common but more expensive outcome.
How to fix it
- Check whether the boiler is in lockout or blocking mode DIY safe
Look at the boiler display. A flashing 'H' symbol alongside 4E / 278 indicates a full lockout — a reset is required. If there is no flashing 'H', the boiler may recover on its own once the fault condition clears. Note this before doing anything else.
- Attempt a controlled reset DIY safe
Press and hold the ▲ and ▼ buttons simultaneously until the 'H' and '!' symbols disappear. Do not press the reset more than two or three times in succession — repeated resets without the underlying fault being resolved can damage the burner ignition transformer. If the fault immediately returns after resetting, stop and call an engineer.
- Check your gas supply DIY safe
Make sure other gas appliances in your home (hob, gas fire) are working normally. If there is no gas at other appliances, contact your gas supplier. A loss of gas supply can cause the boiler to mis-fire and occasionally trigger temperature sensor faults.
- Engineer: inspect the combustion chamber sensor and wiring harness Gas Safe engineer
A Gas Safe engineer should visually check the NTC sensor mounted in or near the combustion chamber for signs of damage, corrosion, or overheating. The wiring harness should be traced along its full run and checked for chafing, melted insulation, or loose terminals.
- Engineer: measure sensor resistance Gas Safe engineer
Using a multimeter, the engineer should measure the sensor's resistance at a known temperature and compare it against the Buderus specification. If the reading is outside the acceptable range, swap the sensor temporarily with a known-good unit and re-test. If the fault clears with the replacement sensor, fit a new original Buderus sensor permanently.
- Engineer: inspect UBA board connectors and pins Gas Safe engineer
With power isolated, the engineer should remove and re-seat the sensor connector at the UBA board, checking for bent pins, corrosion, or debris in the socket. If a pin is bent, it must be carefully realigned before reconnecting.
- Engineer: replace the UBA control board if all else checks out Gas Safe engineer
If the sensor and all wiring test correctly but the fault persists, the UBA board is likely defective. The engineer should arrange replacement using a genuine Buderus part, configuring the new board correctly to the specific boiler model. Signs of board failure include visibly deformed or scorched components, condensation damage, or broken tracks.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
All steps beyond the initial reset require a Gas Safe registered engineer. You can verify your engineer's credentials at gassaferegister.co.uk. Always ask for a gas-safe certificate on completion of any work on the boiler's gas or control circuits.
Parts you may need
- Buderus NTC combustion chamber temperature sensor · from £45
- Sensor wiring harness / loom · from £35
- UBA control board (Buderus UBA 3.x) · from £280
- Connector repair kit / replacement terminals · from £12
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–£320, depending on the underlying cause.
Frequently asked questions
Can I reset a Buderus 4E fault myself?
Yes — you can attempt a reset by holding the ▲ and ▼ buttons simultaneously until the 'H' and '!' indicators clear. However, limit yourself to two or three attempts. If the fault returns immediately or the boiler won't fire, stop resetting and call a Gas Safe engineer. Repeated resets without fixing the root cause can damage the ignition transformer.
How much does it cost to fix a Buderus 4E / 278 fault?
For the most common cause — a failed NTC temperature sensor — expect to pay roughly £120–£250 all in, covering an engineer's callout, the part (around £40–£60), and the labour to fit and test it. If the UBA control board needs replacing, costs typically rise to £300–£450 or more, as the board itself can cost £200–£300 before labour. Note that Buderus is a less common brand in the UK, so parts may occasionally need to be ordered, adding a little time and cost compared with more mainstream brands.
What is the difference between a blocking fault and a lockout on a Buderus boiler?
A blocking fault is temporary: the boiler shuts down but will restart automatically once the triggering condition goes away — no user action is needed. A lockout is more serious: the boiler stays off until someone performs a manual reset. On the display, a lockout is identified by a flashing 'H' symbol alongside the fault code. If 4E / 278 persists long enough (for example, due to a persistent open circuit in the sensor wiring), it escalates from a blocking fault to a full lockout.
Could limescale cause a 4E / 278 fault on my Buderus boiler?
Yes, in hard-water areas. Scale build-up inside the heat exchanger restricts water flow and causes hot spots, which places the combustion chamber temperature sensor under abnormal heat stress. This can produce erratic readings that the UBA board logs as a sensor fault. If your engineer finds scale is a contributing factor, they may recommend a system flush or descale as part of the repair, and fitting an inline scale inhibitor to prevent recurrence.