Buderus 4U 222 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 4U 222 fault code mean?
The 4U fault code (operating code 222) tells you that the boiler's flow temperature sensor — the NTC thermistor that monitors how hot the water leaving the heat exchanger is — has developed a short circuit. Because the PCB can no longer trust the temperature reading it is receiving, the boiler shuts down to protect itself. Initially this appears as a blocking fault, but if the problem persists without being resolved it escalates into a full lockout. The display typically shows '4U' alongside operating code '222', confirming specifically that the supply sensor is affected by a short circuit rather than an open-circuit or out-of-range reading.
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
- Failed NTC flow temperature sensor Common
The NTC thermistor is exposed to repeated heating and cooling cycles throughout its working life. Over time the internal resistance element can degrade and develop a short circuit, causing the PCB to read an implausibly low resistance value. This is the single most frequent cause of 4U 222 on Buderus appliances.
- Damaged or corroded sensor wiring and connectors Common
The wiring loom running from the flow sensor to the PCB can suffer from heat damage, chafing against the casing, or terminal corrosion — any of which can create a low-resistance short between conductors. Intermittent versions of this fault often point here rather than to the sensor itself.
- Faulty PCB or control board Sometimes
If the sensor and its wiring both test correctly, attention turns to the PCB. Moisture ingress, component deformation from overheating, or degraded circuit tracks can cause the board to misread a healthy sensor as a short circuit. PCB faults tend to produce other error codes alongside 4U 222 if the board is failing broadly.
- Electrical supply instability Rare
Significant voltage fluctuations or electrical interference reaching the boiler can disrupt sensor signal readings and trigger a spurious 4U 222 code. This is more likely in properties with an unstable mains supply or where the boiler shares a circuit with equipment that causes electrical noise.
How to fix it
- Reset the boiler (up to two or three attempts) DIY safe
Press and hold the reset button for approximately three seconds. If the boiler fires up and runs normally, the fault may have been a transient electrical glitch. If 4U 222 returns immediately or after a very short run, a component fault is confirmed and further investigation by a Gas Safe engineer is needed. Avoid resetting more than two or three times as repeated resets without fixing the underlying problem can cause further damage.
- Check that the gas and electricity supplies are normal DIY safe
Confirm the gas meter isolation valve is fully open and that other gas appliances in the property are working. Check the boiler's fused spur or isolation switch is on and has not tripped. If a circuit breaker has tripped, reset it once — if it trips again, do not reset it further and call an electrician.
- Have a Gas Safe engineer test the NTC flow temperature sensor Gas Safe engineer
An engineer will disconnect the sensor connector and measure the thermistor's resistance at a known ambient temperature using a multimeter. A short circuit will show near-zero resistance rather than the thousands of ohms expected. If the sensor is confirmed faulty, it should be replaced with a genuine Buderus-approved part — similar-looking sensors from other manufacturers may have different resistance curves or connector types and can introduce new faults.
- Have the engineer inspect the sensor wiring loom and connectors Gas Safe engineer
Even if the sensor itself tests fine, the engineer should trace the wiring back to the PCB, checking for chafed insulation, heat-damaged cables, or corroded push-fit terminals. A damaged section of loom can be repaired or the harness replaced as required.
- Have the PCB inspected or replaced if sensor and wiring are both sound Gas Safe engineer
If the sensor and loom both pass testing, the fault lies within the control board. The engineer can carry out further diagnostics to confirm PCB failure before recommending a replacement. Given the cost of a PCB, a reputable engineer should verify this conclusion before ordering parts. On older appliances, the cost of a new PCB may warrant a conversation about whether boiler replacement is more economical.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All work beyond the homeowner checks above — sensor testing, wiring diagnosis, PCB work, and any reassembly involving sealed waterways or electrical components inside the boiler — must be carried out by a Gas Safe registered engineer who is competent on Buderus appliances. You can find registered engineers at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- Buderus NTC flow temperature sensor (thermistor) · from £28
- Sensor wiring loom / harness · from £35
- Buderus PCB / control board (model-specific) · 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 £120–£280, depending on the underlying cause.
Frequently asked questions
Is the Buderus 4U 222 fault dangerous — can I keep using the boiler?
The boiler will typically shut itself down when this fault appears, so it should not continue operating in an unsafe condition. However, you should not attempt to repeatedly override or reset the lockout to keep the boiler running, as the flow temperature protection exists for good reason. Leave the boiler off and arrange for a Gas Safe engineer to investigate promptly, particularly in cold weather.
What is the difference between fault code 4U and operating code 222 on a Buderus boiler?
Buderus displays often show two pieces of information: a fault category code (here '4U') and a numeric operating code (here '222'). The '4U' category indicates a sensor-related blocking or lockout fault, while '222' pinpoints the specific problem as a short circuit on the supply (flow) temperature sensor. Together they confirm the same root cause, so you do not need to treat them as two separate faults.
How much does it cost to fix a Buderus 4U 222 fault in the UK?
For a straightforward NTC sensor replacement you should typically budget £120–£280 including parts and a Gas Safe engineer's call-out and labour. The sensor itself costs around £15–£40 as a part; the majority of the bill is the engineer's time. If the fault has escalated to a PCB problem, costs rise considerably — a new PCB fitted can run to £350–£500 or more — but this is less common and an engineer should confirm the diagnosis before ordering a board.
Can I replace the flow temperature sensor myself to save money?
No. Although the sensor is an inexpensive part, replacing it involves accessing internal boiler components, disconnecting sealed electrical connections, and potentially draining part of the system. This work must be carried out by a Gas Safe registered engineer. Using an unqualified person to work on a gas appliance is illegal in the UK, can void your home insurance, and puts your household at risk. The labour cost for a sensor swap is modest — it is not a lengthy job for a competent engineer.