Buderus 9L 238 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 9L 238 fault code mean?
The 9L fault family on Buderus boilers relates to an electrical problem with the gas valve circuit — specifically, the boiler's control electronics cannot confirm that the gas valve coil or its associated wiring is functioning correctly, so the boiler shuts down as a safety precaution. The three-digit sub-code 238 narrows the diagnosis toward a fault with the UBA 3 (Universal Burner Automator) or KIM (combustion identification module) rather than a simple cable break. A closely related sub-code, 9L 234, is specifically described by Buderus as a faulty gas burner fitting cable connection; if you see 234 rather than 238 on your display, the wiring harness to the valve is the more likely culprit before the UBA board itself. In both cases the boiler locks out until the underlying electrical issue is resolved. This is not a fault a homeowner can repair — all diagnostic and corrective work must be carried out by a Gas Safe registered engineer.
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
- Loose or corroded wiring connections at the gas valve Common
Over time, vibration from normal boiler operation can gradually loosen the plug-in connectors between the gas valve and the UBA. Corrosion at these terminals can also increase resistance to the point where the UBA detects an out-of-range signal and triggers the lockout. This is the most frequent root cause and is often resolved by a simple reconnection or terminal clean.
- Defective gas valve coil Common
The solenoid coil inside the gas valve has a nominal resistance of around 18–55 ohms. If the coil has degraded or developed an open circuit, the UBA will not see the expected electrical response and will flag the 9L fault. Coil failure is more common on boilers that have seen many years of service or repeated thermal cycling.
- Faulty UBA 3 control board Sometimes
When wiring and the valve coil both test within specification, the UBA board itself is the next suspect. The UBA is responsible for interpreting sensor and coil signals; if its internal circuitry has failed it can misread a healthy gas valve and report a 9L 238 fault. Buderus's own guidance for the 238 sub-code specifically lists UBA replacement as the prescribed corrective action. Note that replacing the UBA resets all user-defined settings on controllers such as the RC30 and RC35.
- Electrical interference or voltage spikes on the supply Sometimes
Voltage peaks on the mains supply can cause transient faults that confuse the UBA's signal monitoring. In some cases a single reset clears the fault with no further action needed; if the code returns repeatedly without any hardware fault being found, the quality of the incoming electrical supply should be investigated.
- Faulty main PCB Rare
Because it is the PCB that ultimately generates and displays fault codes, a failing PCB can point the finger at gas valve components even when those components are healthy. This is a less common scenario but should be considered if valve, wiring, and UBA all test correctly.
How to fix it
- Do not reset the boiler more than two or three times DIY safe
Pressing reset repeatedly when the burner is failing to start risks damaging the ignition transformer. Attempt a single reset; if the fault returns immediately, stop and move on to calling an engineer rather than cycling the reset button further.
- Check that your gas supply is available DIY safe
Confirm that other gas appliances in the property (hob, gas fire) are working normally. If there is no gas elsewhere, contact your gas supplier — this is not a boiler fault. If gas is available to other appliances, the issue is within the boiler itself.
- Inspect and reseat the gas valve wiring connectors — engineer only Gas Safe engineer
A Gas Safe engineer will disconnect power, remove the boiler casing, and carefully unplug and re-seat the connector block at the gas valve. They will visually inspect each terminal for signs of burning, corrosion, or pushed-back pins and clean or replace terminals as required.
- Measure gas valve coil resistance — engineer only Gas Safe engineer
Using a calibrated multimeter set to the lowest ohm range, the engineer will measure the resistance across the valve coil terminals. A reading outside the 18–55 ohm range indicates a defective coil. They will also check each wire in the loom for continuity and test between wires for any short circuits.
- Perform live voltage measurement at the gas valve — engineer only Gas Safe engineer
While the fault operating code is active, the engineer will use a volt-ohm meter to confirm that 20–24 V DC is present at the gas valve plug — between the central and left contacts, and between the central and right contacts. Absence of voltage at this point shifts suspicion toward the UBA or PCB rather than the valve itself.
- Trial substitution of the UBA board — engineer only Gas Safe engineer
If wiring and valve coil are both confirmed good, the engineer may temporarily connect a known-working UBA to determine whether the original board is the source of the fault. If the substitute UBA clears the fault, the original unit should be replaced. The engineer will need to restore all user settings on the room controller after fitting a new UBA.
- Replace defective components as indicated — engineer only Gas Safe engineer
Depending on findings, the engineer will replace the gas valve coil or complete valve assembly, the UBA 3 board, or — if all other components test correctly — the main PCB. Only Gas Safe registered engineers may carry out work on gas components.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All of the diagnostic and repair steps above require a Gas Safe registered engineer. If you have not already booked one, do so now. Have your boiler model number and the full fault code (9L 238 or 9L 234) ready when you call so the engineer can bring likely replacement parts.
Parts you may need
- Gas valve assembly (Buderus GB162/GB172 compatible) · from £130
- UBA 3 Universal Burner Automator board · from £200
- Gas valve wiring loom / connector set · from £25
- Main PCB (Buderus Logamax Plus series) · 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Buderus 9L 238 fault myself?
No. The 9L 238 fault involves the gas valve circuit and associated control electronics. Any work on gas components is legally required to be carried out by a Gas Safe registered engineer in the UK. You can safely check that your gas supply is working and attempt a single boiler reset, but beyond that the diagnosis and repair must be left to a qualified professional.
What is the difference between fault codes 9L 238 and 9L 234?
Both codes belong to the same 9L fault family relating to the gas valve electrical circuit. Buderus describes 9L 234 specifically as a faulty cable connection at the gas burner fitting — so if your display shows 234, the wiring harness or connectors are the most likely culprit. Sub-code 238 points more toward a defect in the UBA 3 control board or KIM module itself, though wiring should always be ruled out first before replacing the UBA.
How much does it cost to fix a Buderus 9L 238 fault in the UK?
Most repairs fall in the range of £150–£400 including parts and labour. Wiring reconnection or a terminal clean at the lower end, a gas valve replacement typically £180–£280, and a UBA board replacement around £250–£400. If the main PCB turns out to be the root cause, costs can rise higher — typically £300–£500 or more — but PCB failure is a less common outcome for this particular fault code.
Will resetting my Buderus boiler clear the 9L 238 fault?
Occasionally a transient voltage spike can trigger a 9L 238 code that clears after a single reset. However, you should not press reset more than two or three times in succession — doing so risks damaging the ignition transformer. If the fault returns after one or two resets, stop resetting and contact a Gas Safe engineer, as repeated resets will not resolve an underlying wiring or component fault and may cause additional damage.
Does replacing the UBA board erase my boiler settings?
Yes. Buderus confirms that fitting a new UBA 3 board resets all user-defined parameters stored in the control unit, including any settings programmed via a room controller such as the RC30 or RC35. Ask your engineer to note your current settings — heating schedules, hot water temperatures, and so on — before replacing the board so they can be reprogrammed afterwards.