Buderus 4Y Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 4Y fault code mean?
Fault code 4Y on a Buderus boiler indicates that the flow temperature sensor has developed an open-circuit fault — in other words, the signal from the sensor has been completely interrupted. The flow sensor (an NTC thermistor) measures the temperature of water as it leaves the heat exchanger, and the boiler's controls rely on this reading to decide when to fire the burner and when to shut it down. With no valid temperature signal, the boiler cannot operate safely and will shut itself down. Sub-codes 4Y 351 and 4Y 223 both point to this same open-circuit break fault. The code may appear as a steady display (a blocking fault, where the boiler pauses but does not need a manual reset) or as a flashing display (a locking fault, where a manual reset is required after the underlying cause has been fixed). Note: the closely related code 4U describes the opposite condition — a short-circuit fault on the same flow sensor — so if you see 4U rather than 4Y, the sensor wiring has shorted rather than broken.
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 thermistor (flow sensor internally broken) Common
The NTC thermistor itself has failed internally — the resistive element inside has broken, creating an open circuit. This is the most frequent cause of a 4Y code. A functioning NTC thermistor has a predictable resistance at any given temperature; when the element breaks, resistance becomes infinite and the boiler control board receives no usable signal.
- Loose or corroded connector at the sensor plug Common
Over time, the electrical connector joining the sensor to the wiring loom can work loose or become corroded, interrupting the signal. This is particularly common in older boilers or those in damp plant rooms. The fault may appear intermittently before becoming permanent, and the sensor itself may still be electrically sound.
- Damaged or broken wire in the sensor cable harness Sometimes
A break in one of the wires running between the flow sensor and the PCB will produce the same open-circuit reading as a failed sensor. Wires can chafe against pipework, be pinched during previous servicing, or develop hairline fractures from repeated heating and cooling cycles.
How to fix it
- Check whether the boiler is in a locking (flashing) or blocking (steady) fault state DIY safe
Look at the boiler display. If the 4Y code is flashing, the boiler has locked out and will need a manual reset once the fault is resolved. If it is displayed steadily, it is a blocking fault and the boiler may restart on its own once conditions improve. Do not reset a locking fault more than two or three times without addressing the cause, as repeated resets without a fix can mask underlying damage.
- Press the reset button to attempt a restart (locking fault only) DIY safe
On most Buderus models, hold the reset button for roughly two to three seconds. If the boiler fires up and runs normally, monitor it closely over the next hour or so. If code 4Y returns immediately or shortly afterwards, a component needs professional attention and further resets will not help.
- Inspect the sensor wiring and connector for obvious damage Gas Safe engineer
A Gas Safe engineer will visually check the wiring harness running from the flow sensor to the PCB, looking for chafed insulation, pinched cables, or a connector that has pulled partially free. Loose connectors are reseated and any corroded pins cleaned or replaced. This takes only a few minutes and costs nothing in parts if this turns out to be the sole issue.
- Test the flow sensor resistance with a multimeter Gas Safe engineer
The engineer will disconnect the NTC thermistor and measure its resistance at the ambient temperature of the boiler. NTC sensors follow a known resistance-temperature curve — a reading of infinite resistance (open circuit) or one that falls well outside the expected range confirms the sensor has failed and must be replaced.
- Replace the flow temperature sensor if confirmed faulty Gas Safe engineer
If the sensor has failed, the engineer will fit a replacement NTC thermistor. A small amount of thermal paste should be applied between the sensor body and the pipe surface to ensure accurate temperature transfer. Buderus-compatible sensors are inexpensive; using a quality replacement — ideally an original Buderus part — ensures reliable operation going forward.
- Inspect or replace the cable harness if the sensor tests correctly Gas Safe engineer
When the sensor itself measures within specification, the fault lies in the wiring. The engineer will trace the harness for breaks or damaged sections, repairing or replacing the affected wiring. In some cases the full loom section between the sensor and the PCB is replaced to ensure a lasting repair.
- Reset the boiler and verify the fault has cleared Gas Safe engineer
After completing the repair, the engineer resets the boiler, allows it to complete a full heating cycle, and confirms that fault code 4Y does not reappear. System pressure, flue output, and general boiler operation should also be checked before the job is signed off.
- Contact a Gas Safe registered engineer if the fault persists or you have not already done so Gas Safe engineer
All diagnostic and repair work on a Buderus boiler must be carried out by a Gas Safe registered engineer. If you have attempted a reset and the code has returned, do not continue resetting — call a qualified engineer. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Buderus flow temperature NTC thermistor sensor · from £25
- Sensor wiring harness / connector repair kit · from £18
- Thermal paste (for sensor fitting) · from £6
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 £90–£220, depending on the underlying cause.
Frequently asked questions
What is the difference between Buderus fault codes 4Y and 4U?
Both codes relate to the flow temperature sensor, but they describe opposite electrical faults. Code 4Y means the sensor circuit has broken open — there is no electrical path and the boiler receives no temperature signal at all. Code 4U means the same sensor circuit has shorted — the wiring or sensor has developed an unintended direct connection, sending an incorrect signal. The fix for both typically involves inspecting the sensor and its wiring, but a short (4U) and a break (4Y) are diagnosed slightly differently with a multimeter.
Can I reset my Buderus boiler myself when it shows 4Y?
Yes, you can try a single reset — hold the reset button for two to three seconds. If the boiler runs normally afterwards and the code does not return, it may have been a temporary glitch. However, if 4Y comes back immediately or within a short time, do not keep resetting it. Repeated resets without fixing the underlying fault can cause further wear and will not resolve a failed sensor or broken wire. At that point, you need a Gas Safe registered engineer.
How much does it cost to fix a Buderus 4Y fault code in the UK?
For most homeowners, the total bill falls somewhere between £90 and £220. If the issue is simply a loose connector, the cost may be at the lower end — mainly covering the engineer's call-out and a short labour time with no parts needed. If the NTC thermistor itself needs replacing, the sensor typically costs £15–£35 and the total including labour is usually £120–£200. Out-of-hours or emergency call-outs will cost more. In the unlikely event the PCB has been damaged by the open-circuit condition, that is a much larger repair — your engineer will advise if this is suspected.
What are the sub-codes 4Y 351 and 4Y 223 on a Buderus boiler?
Buderus boilers can display extended codes that combine the main fault number with a sub-code. Both 4Y 351 and 4Y 223 point to the same underlying problem — an open-circuit interruption in the flow temperature sensor circuit. The sub-code identifies which internal diagnostic path flagged the fault, but the diagnosis and repair procedure is the same in both cases: check the sensor, its connector, and the wiring harness, then replace whichever component has failed.