Buderus 2U 213 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 2U 213 fault code mean?
The Buderus 2U 213 fault code is made up of two parts: the fault category prefix '2U', which covers heating system operational faults on EMS-controlled Buderus boilers, and the specific sub-code '213', accessed by pressing the Service button on the BC10 controller. Together they tell you that the boiler has detected an unusually large temperature gap between the flow and return sensors — a clear sign that water is not circulating properly through the system. When water moves too slowly or not at all, heat builds up rapidly in the heat exchanger rather than being distributed around your home, and the boiler shuts down temporarily as a protective measure. This type of fault is classified as a blocking fault, meaning the boiler will attempt to restart automatically once conditions improve, but if the underlying cause is not addressed the fault will keep returning.
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
- Seized or failing circulator pump Common
The circulator pump is responsible for pushing hot water from the boiler around the entire heating circuit. If it has seized, is running below speed, or has failed outright, water stagnates in the heat exchanger and the flow-to-return temperature difference spikes very quickly — a textbook trigger for fault 213. Pump failure is the single most common root cause of this code.
- Closed or partially closed isolation valves Common
Service valves or isolation valves on the heating circuit that are partially or fully closed will strangle water flow just as effectively as a failed pump. Even one closed valve on a main flow or return pipe can cause the temperature differential to exceed the boiler's threshold. These are easy to overlook after maintenance work.
- Air locks in the heating system Common
Pockets of trapped air can form in radiators, pipework, or around the pump itself. Air does not conduct heat the way water does, so wherever it accumulates it creates a dead zone that disrupts circulation and can cause localised overheating. This is particularly common after system pressure has been lost and then topped up, or after radiators have been bled.
- Sludge or limescale restricting the heat exchanger Sometimes
In hard-water areas or systems that have not been power-flushed in many years, magnetite sludge and limescale can partially block the internal passages of the heat exchanger. Water struggles to pass through, the boiler heats a small volume of water very quickly, and the return temperature lags well behind — generating exactly the high differential that triggers 2U 213.
- Incorrect pump output or characteristic settings Sometimes
On some Buderus installations the circulator pump output level or its operating curve can be configured via the BC20 controller. If these settings do not match the system's actual requirements — for example after a pump swap or a system extension — the pump may be running at too low a speed to maintain adequate flow, even though mechanically it is working correctly.
- Faulty flow or return temperature sensor Rare
If either the NTC flow sensor or the NTC return sensor is giving inaccurate readings due to a wiring fault, corrosion, or a failing sensor element, the boiler's UBA control module may calculate a temperature differential that does not reflect real conditions. In this scenario the circulation itself may be fine but the boiler still triggers the fault. A prolonged blocking fault that does not clear spontaneously is one indicator that a sensor rather than actual flow is to blame.
How to fix it
- Check system pressure on the boiler's pressure gauge DIY safe
The filling loop pressure when the system is cold should sit between 1.0 and 1.5 bar. If it reads below 1.0 bar, top up via the filling loop (usually a silver or grey braided hose beneath the boiler with one or two valves). Open the valve(s) slowly until the needle reaches around 1.2 bar, then close them firmly. Low pressure alone can impair pump performance and contribute to poor circulation.
- Check that all radiator TRVs and isolation valves are fully open DIY safe
Walk around each radiator and make sure the thermostatic radiator valves (TRVs) are turned up rather than closed off. Also check any isolation valves on the heating pipework — typically gate or ball valves near the boiler — to confirm they are fully open (handle in line with the pipe for a ball valve). A single closed valve can be enough to trigger fault 213.
- Bleed any air from the radiators DIY safe
Starting from the ground floor and working upwards, use a radiator bleed key to open the bleed valve on each radiator by half a turn. Hold a cloth underneath. When water dribbles out steadily instead of air hissing, close the valve. Re-check the system pressure after bleeding and top up again if it has dropped below 1.0 bar, as releasing air will lower the reading slightly.
- Reset the boiler DIY safe
Once you have checked pressure, valves, and bled the radiators, press the Reset button on the boiler control panel to clear the fault. Allow the boiler to fire up and monitor it for 10–15 minutes. If the fault clears and the heating runs normally, keep an eye on it over the next day or two. Do not press Reset more than two or three times in succession — repeated resets without resolving the underlying cause can cause additional stress to ignition components.
- If the fault returns, have a Gas Safe engineer inspect the circulator pump Gas Safe engineer
A Gas Safe registered engineer will check whether the pump is spinning freely, drawing the correct current, and delivering adequate flow rate. A pump that is audibly humming but not moving water, or one that is completely silent, almost certainly needs replacing. The engineer will also verify the pump output settings in the BC20 controller to ensure they match the system's requirements.
- Have the engineer check the UBA control module sensor connections Gas Safe engineer
The Universal Burner Automation (UBA) module, located at the lower left of the boiler beneath the cover, manages sensor inputs including flow and return temperatures. The engineer will check the wiring harness and connectors at the UBA, and test the NTC flow and return sensors for correct resistance values across their operating temperature range. A short circuit or open circuit in either sensor or its cable will cause a false differential reading.
- Have the engineer assess the heat exchanger for sludge or scale Gas Safe engineer
If pump and sensor checks come back normal, restricted flow through a fouled heat exchanger becomes the likely culprit. The engineer may carry out a chemical flush or recommend a full power flush of the system. Inhibitor should be added after any flush to slow future corrosion and sludge build-up.
- Call a Gas Safe registered engineer if any of the above steps do not resolve the fault Gas Safe engineer
Persistent 2U 213 faults that do not respond to homeowner checks or initial engineer diagnostics may point to a failing pump needing replacement, a defective temperature sensor, or a heavily scaled heat exchanger. Only a Gas Safe registered engineer should carry out component replacement or gas-side work on the boiler. You can find one at gassaferegister.co.uk.
Parts you may need
- Circulator pump (compatible replacement) · from £95
- NTC flow temperature sensor · from £25
- NTC return temperature sensor · from £25
- System inhibitor (1 litre, e.g. Fernox F1) · from £15
- BC10 / BC20 controller module · from £120
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 £150–£450, depending on the underlying cause.
Frequently asked questions
Will the boiler come back on by itself after a 2U 213 fault?
Because 2U 213 is classified as a blocking fault rather than a full lockout, the boiler is designed to attempt an automatic restart once the temperature differential drops back within limits. In practice this means it may recover on its own if the cause was a brief air pocket or a transient flow restriction. However, if the same fault keeps reappearing — particularly within the same heating session — something is genuinely wrong and a Gas Safe engineer needs to investigate rather than relying on auto-recovery.
Could closed TRVs really cause fault 2U 213?
Yes, and it is more common than people expect. If most radiators in the house have TRVs turned to a low setting or fully closed — for example in unused rooms during summer — the total flow resistance in the circuit can rise dramatically. The pump has to push water through a much smaller effective circuit, flow slows, and the heat exchanger temperature climbs faster than the return. Opening more TRVs or fitting a bypass valve gives the system the minimum flow path it needs and often resolves the fault without any parts being replaced.
How much does it cost to replace the pump on a Buderus boiler?
Pump replacement including parts and labour typically costs between £200 and £400 with an independent Gas Safe engineer. The price varies depending on the specific pump model fitted, your location (London and the South East tend to run 20–30% higher), and whether access is straightforward. If you also need a power flush because sludge contributed to the pump's early failure, budget an additional £350–£500 for that. Heat exchanger replacement is a much larger job and can cost £600–£1,000 or more, but this is relatively uncommon for fault 213 alone and is not reflected in the typical repair range above.
How do I access the 213 sub-code on my Buderus boiler?
On Buderus EMS-controlled boilers, the display first shows the fault category — in this case '2U'. To see the three-digit sub-code that pinpoints the specific fault, press the Service button on the BC10 or BC20 controller. The display will then show '213', confirming this is the flow/return differential fault. If you see a different three-digit number after '2U', the fault has a different cause and you should look up that specific sub-code in the Buderus documentation or consult a Gas Safe engineer.