Buderus 0Y 277 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 0Y 277 fault code mean?
The Buderus 0Y fault code (displayed as '0Y 277' on many Buderus controllers) tells you that the boiler has detected a flow temperature above its safe operating threshold and has shut itself down as a precautionary measure. In plain terms, the water circulating through the heating system is running hotter than it should, so the boiler has temporarily blocked itself to prevent damage or a safety hazard. The '277' is a sub-code that accompanies 0Y on Buderus digital controllers to give additional diagnostic context — if you see '0Y 277' on your display, both numbers together are referring to the same overtemperature condition. The fault can manifest either as the actual flow temperature exceeding the programmed set-point, or as the domestic hot water temperature sensor reporting an abnormally high reading. Either way, the root cause is the same: heat is building up somewhere it should not be, and the boiler needs attention before it will run normally 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
- Circulation pump failure or blockage Common
If the central heating pump is seized, running too slowly, or obstructed by debris, hot water cannot move around the system fast enough to dissipate heat. The water trapped near the burner rapidly climbs in temperature and triggers the 0Y lockout. Pump bearings can seize after prolonged operation, and sludge in an unmaintained system can clog the impeller.
- Limescale or sludge in the heat exchanger Common
In hard-water areas of the UK, limescale deposits build up inside the heat exchanger over time, narrowing the waterways and reducing the flow rate. Similarly, black iron oxide sludge from corroding radiators can accumulate and cause hot spots. Both restrict heat transfer and push flow temperatures well above normal operating levels.
- Faulty or displaced hot water temperature sensor (NTC thermistor) Common
The NTC thermistor sends a resistance reading to the PCB that corresponds to the current water temperature. If the sensor has drifted out of its correct mounting position, or if the thermistor element itself has degraded, it may report a temperature that is far higher than reality — or it may fail to detect that the water is genuinely overheating. Either scenario can trigger the 0Y code.
- Low system pressure reducing circulation Sometimes
When the system pressure drops below around 0.5–0.8 bar, there is simply not enough water volume moving through the circuit to carry heat away from the heat exchanger efficiently. This allows localised overheating near the burner and can cause the flow temperature sensor to flag an overtemperature condition.
- Airlocks in the heating circuit Sometimes
Pockets of trapped air act as barriers to water flow, creating uneven circulation and hot spots in parts of the system. Radiators that are cold at the top while the boiler is running are a tell-tale sign. These air pockets can prevent adequate cooling of the heat exchanger and contribute to rising flow temperatures.
- Faulty pressure relief valve (PRV) Rare
The PRV is designed to release excess pressure before it becomes dangerous. If it has failed in the closed position, the system cannot vent excess pressure, which can allow temperatures to climb unchecked. A PRV that is visibly weeping or discharging water continuously may also indicate a wider pressure management problem.
How to fix it
- Reset the boiler — once or twice only DIY safe
Locate the reset button on the Buderus control panel (usually marked with a flame or reset symbol). Press and hold it for a few seconds until the display clears. Give the boiler five minutes to attempt a restart. If the 0Y 277 code returns immediately, do not keep pressing reset — repeated resets without addressing the underlying fault can cause further damage, and manufacturers recommend no more than two or three attempts before investigating properly.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler front or pipework. Cold system pressure should sit between 1.0 and 1.5 bar. If it reads below 1 bar, the low water volume may be contributing to the overtemperature. Locate the filling loop (usually a braided flexible hose beneath the boiler with one or two small valves) and slowly open the valves to admit mains water, watching the gauge rise. Stop at 1.2–1.3 bar, close the valves fully, and try resetting the boiler. If pressure drops again within a few days, you likely have a leak — call an engineer.
- Bleed the radiators to remove air pockets DIY safe
Turn the heating off and allow the system to cool for 30 minutes. Starting on the top floor, use a radiator bleed key to open each bleed valve slightly — hold a cloth underneath to catch drips. When a steady stream of water (rather than air or spluttering) emerges, close the valve. Work your way down through every radiator and towel rail. Once finished, recheck the system pressure and top up to 1.2 bar if needed, then reset the boiler.
- Listen to the circulation pump for signs of failure DIY safe
With the heating running (or attempting to run), place your hand carefully near the pump body — it should feel warm and you should hear or feel a gentle hum of operation. Complete silence, a loud grinding noise, or an unusually hot pump casing all suggest the pump is struggling. You can report these observations to an engineer, but do not attempt to open or replace the pump yourself — this requires draining the system and is not a DIY task.
- Check whether a frozen or blocked condensate pipe is contributing DIY safe
In cold weather, the plastic condensate pipe that runs to an external drain can freeze solid, which sometimes causes secondary faults and sensor errors. Trace the pipe (usually white plastic, 21–22 mm diameter, exiting the boiler and running to an outside drain). If it feels solid or you can see ice, thaw it gently using warm — not boiling — water poured along its length. Once clear, reset the boiler.
- Have the temperature sensor position checked and tested Gas Safe engineer
A Gas Safe engineer should verify that the hot water or flow temperature sensor is correctly seated in its pocket and making proper contact with the pipework. They will use a multimeter to measure the sensor's resistance at a known temperature and compare it against the manufacturer's specification. A sensor reading outside tolerance should be replaced.
- Have the circulation pump inspected and replaced if necessary Gas Safe engineer
An engineer can assess pump output, check for seized bearings, and test electrical continuity. If the pump is confirmed faulty, they will drain the relevant section of the system, fit a compatible replacement pump, refill, bleed, and repressurise before testing operation. Pump replacement on a Buderus boiler is a straightforward job for a competent engineer.
- Arrange a power flush or heat exchanger descale if sludge or limescale is suspected Gas Safe engineer
If the engineer identifies restricted flow caused by sludge or limescale — evidenced by slow-warming radiators, discoloured system water, or visible deposits — a power flush using a specialist machine and chemical cleanser is the recommended solution. For localised limescale within the heat exchanger, a descaling chemical treatment may be used. After flushing, fitting a magnetic system filter (such as a Magnaclean or equivalent) will help prevent a recurrence.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the 0Y 277 code continues after the homeowner checks above, the fault lies with a component that must be diagnosed and repaired by a qualified professional. This could include the NTC thermistor, circulation pump, PCB, PRV, or the heat exchanger itself. Do not attempt further resets or run the boiler while the overtemperature fault is active — sustained high temperatures can cause permanent damage to internal components.
Parts you may need
- NTC thermistor / flow temperature sensor · from £18
- Circulation pump (compatible replacement) · from £85
- Pressure relief valve (PRV) · from £22
- Magnetic system filter (e.g. Magnaclean 22mm) · from £55
- Filling loop assembly · from £15
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–£400, depending on the underlying cause.
Frequently asked questions
Is the Buderus 0Y 277 fault dangerous — can I keep using the boiler?
You should not attempt to run the boiler while the 0Y 277 fault is active. The code means the boiler has detected dangerously high water temperatures and has shut itself down deliberately. Bypassing this safety shutdown or repeatedly resetting without fixing the root cause risks damage to the heat exchanger, seals, and other internal components. Switch the boiler off and arrange for an engineer to investigate.
What is the difference between the '0Y' part and the '277' sub-code on the Buderus display?
On Buderus boilers fitted with digital controllers, faults are displayed as a two-part code. The first part (0Y) identifies the fault category — in this case, an overtemperature condition. The second number (277) is a sub-code that provides additional diagnostic detail about which sensor or circuit triggered the shutdown. Both together (0Y 277) point to the same underlying issue: flow or hot water temperatures running above the safe limit. You may see 0Y on its own on older or simpler displays, but the meaning is the same.
How much does it typically cost to fix a Buderus 0Y 277 fault in the UK?
For the most common causes — a faulty temperature sensor or a failing circulation pump — most homeowners pay somewhere between £120 and £400 including parts and labour. A sensor replacement is usually at the lower end (around £100–£200), while pump replacement tends to fall in the £200–£400 range. A power flush to clear sludge or limescale typically costs £400–£600 and is usually a separate job. In rare cases where the heat exchanger itself needs replacing, costs can rise significantly — often to the point where a full boiler replacement becomes worth considering instead.
How can I prevent the 0Y overtemperature fault from coming back?
The most effective preventive measures are: an annual boiler service by a Gas Safe engineer (who will check sensor positions, pump operation, and system water quality); fitting a magnetic filter to the central heating pipework to capture rust and sludge particles before they accumulate; monthly pressure checks to keep the system between 1.0 and 1.5 bar; and — particularly in hard-water areas — periodic use of a scale inhibitor or a one-off power flush if the system water is already contaminated.