Buderus 4Y 223 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 4Y 223 fault code mean?
The Buderus 4Y 223 fault indicates that the boiler's flow temperature sensor has developed an open circuit — meaning the electrical signal from the sensor has been interrupted or broken entirely. The flow sensor (an NTC thermistor) sits at the boiler's heat exchanger outlet and continuously feeds temperature data to the control board so the burner can fire and shut off at the right moments. When this circuit breaks, the boiler loses the ability to monitor flow temperature safely. Initially this appears as a blocking fault, displayed permanently on the screen rather than flashing. If the problem persists without being resolved, it can escalate to a full lockout fault. Note how this differs from the closely related code 4U 222: that code points to a short circuit in the same sensor, whereas 4Y 223 is specifically an open circuit or break in the sensor circuit — two different electrical failure modes requiring the same professional attention but potentially different diagnoses.
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
- Broken or degraded NTC flow temperature sensor Common
The thermistor element itself has failed internally, creating an open circuit. Repeated heating and cooling cycles cause the fine wires inside the sensor to fatigue and eventually fracture. This is the most common root cause when the wiring and connectors appear undamaged on inspection.
- Loose, corroded, or disconnected wiring connector Common
The connector linking the flow sensor to the boiler's control board can work loose over time, or corrosion can build up on the terminals and break the electrical path. The sensor itself may still be functional — the open circuit is at the connection point rather than in the sensor body.
- Damaged sensor cable Sometimes
Physical damage to the cable running between the sensor and the PCB — caused by chafing against pipework, pinching during a previous service, or heat damage — can sever the circuit. A visual inspection of the cable route often reveals the problem area.
- Age-related thermal fatigue Sometimes
In older Buderus boilers, years of thermal expansion and contraction gradually weaken sensor components and solder joints. Even without obvious physical damage, the internal resistance of an aged sensor can drift out of range or fail completely, triggering the 4Y 223 code.
- Control board (PCB) fault affecting sensor input Rare
In rare cases, the fault lies not with the sensor or its wiring but with the input circuit on the PCB itself, which may no longer correctly read the sensor signal. This is typically only diagnosed after the sensor and wiring have been confirmed as good.
How to fix it
- Check whether the boiler is displaying a permanent (non-flashing) fault or a flashing lockout DIY safe
A permanent, non-flashing 4Y 223 display is a blocking fault — the boiler has restricted operation but may not have fully shut down yet. A flashing display indicates a full lockout. Understanding this helps you communicate accurately with an engineer and tells you whether a reset is even required at this stage.
- Attempt a single boiler reset DIY safe
Press and hold the Reset button for approximately 2–3 seconds as described in your boiler's user guide. Do this no more than once or twice. If the fault returns immediately or very quickly, do not keep pressing Reset — doing so more than three times in quick succession can damage the ignition transformer. A recurring fault means a component needs physical inspection by a professional.
- Check your gas supply is live DIY safe
Confirm that other gas appliances in the property (hob, gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier before calling an engineer. A temporary gas interruption can occasionally trigger sensor-related faults that clear once supply is restored.
- Do not attempt to access the flow sensor or wiring yourself Gas Safe engineer
The flow temperature sensor is located inside the boiler casing, directly connected to the heat exchanger pipework and the control board. Testing sensor resistance with a multimeter, disconnecting connectors, and replacing the sensor all require the boiler casing to be opened and work to be carried out on internal components. This must only be done by a Gas Safe registered engineer.
- Arrange for a Gas Safe registered engineer to inspect and test the flow sensor circuit Gas Safe engineer
The engineer will check the NTC sensor's resistance value against the expected range for the water temperature present, inspect the wiring loom and connector for corrosion or damage, and confirm whether the fault lies in the sensor, the cable, or the PCB input. They should also check system operating pressure, bleed the system if necessary, and verify the circulation pump is functioning correctly, as poor flow can exacerbate sensor issues.
- Replace the flow temperature sensor (and apply thermal paste during installation) Gas Safe engineer
If the sensor is confirmed faulty, the engineer will fit a replacement — ideally a genuine Buderus NTC sensor for the specific model. Correct installation includes applying a small amount of thermal paste between the sensor body and the pipe surface to ensure accurate heat transfer and reliable readings. Using a non-OEM sensor may cause ongoing inaccuracies.
- Request a post-repair system check Gas Safe engineer
Once the sensor is replaced and the fault cleared, ask the engineer to confirm the boiler runs through a full heat cycle without the fault returning, check that flow and return temperature readings are behaving correctly, and verify system pressure is between 1–1.5 bar when cold. This reduces the risk of the fault re-appearing shortly after the visit.
Parts you may need
- Buderus NTC flow temperature sensor (thermistor) · from £35
- Sensor wiring harness / connector loom · from £25
- Thermal paste (sensor installation) · from £5
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
What is the difference between Buderus fault codes 4Y 223 and 4U 222?
Both codes relate to the flow temperature sensor, but they point to opposite electrical failures. Code 4U 222 indicates a short circuit — the sensor circuit has too low a resistance, often caused by moisture ingress or a sensor shorted against pipework. Code 4Y 223 indicates an open circuit — the circuit has effectively been broken and no signal is getting through at all. The fix for both involves inspecting the same sensor and wiring, but the test readings the engineer takes will differ, and the root cause can vary.
Can I keep using my Buderus boiler while it is showing fault code 4Y 223?
Not reliably, and not safely in the long run. While a blocking fault may allow limited boiler operation in the short term, the control board cannot properly regulate the burner without accurate flow temperature data. This risks the boiler overheating or cycling erratically, which puts stress on the heat exchanger and pump. You should get it assessed by a Gas Safe engineer as soon as practically possible rather than running the boiler indefinitely with the fault active.
How much does it typically cost to fix the 4Y 223 fault on a Buderus boiler in the UK?
For most households this fault works out at roughly £120 to £280 all-in, covering the engineer's call-out, labour, and the replacement NTC sensor (which is an inexpensive part, typically £15–£50 depending on the model). Costs at the higher end usually reflect more complex wiring faults or a second visit. In rare cases where the PCB input circuit has failed, repair costs can rise significantly — potentially £350 or more — but this is an uncommon outcome once the sensor and wiring have been ruled out first.
How long does it take to replace a Buderus flow temperature sensor?
For an experienced engineer familiar with Buderus boilers, swapping an NTC flow sensor is generally a straightforward job that can be completed within an hour, assuming the correct part is available. The engineer will need to isolate the boiler, drain a small amount of water from the relevant circuit in some installations, fit the new sensor with thermal paste, reconnect the wiring, and run a test cycle. Most visits for this fault are resolved in a single call-out.