Worcester Bosch D3 232 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch D3 232 fault code mean?
The D3 fault code on a Worcester Bosch boiler means the boiler has locked out because an external switching contact has opened. The three-digit cause code 232 tells you specifically that this is linked to an external temperature limiter or safety thermostat that has detected a potentially unsafe condition and broken its circuit to shut the boiler down. In plain terms, a safety device outside the boiler's main body — typically a high-limit thermostat on an underfloor heating manifold, a cylinder stat, or a safety temperature limiter — has tripped or failed, and the boiler's control board has responded by locking out rather than risking an overheat. The boiler will display a flashing fault code alongside a static cause code (232), and it will need a manual reset once the underlying problem has been resolved.
General guidance only — not a substitute for professional advice. 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
- Tripped external temperature limiter or safety thermostat Common
The most common trigger. A high-limit thermostat — fitted to an underfloor heating manifold, a hot water cylinder, or the boiler flow pipe — has reached its trip temperature and broken the switching circuit. This can happen after a genuine overheat event or because the limiter itself is ageing and tripping at lower temperatures than its rating. The limiter must be inspected, and if the cause of any overheating identified, before it is reset or replaced.
- Circulation problem causing elevated temperatures Common
If the system pump is weak, blocked with sludge, or failing, water circulates too slowly through the heat exchanger. Temperatures rise until the safety limiter trips. Low system pressure (below 1 bar) can also restrict flow enough to trigger the limiter. This is the underlying root cause behind many limiter trips and needs to be resolved alongside any limiter work.
- Wiring fault or loose connection on the limiter circuit Sometimes
The boiler's control board reads an open circuit from the external switching input. A loose terminal, corroded connector, or damaged wire between the limiter and the PCB can mimic a genuine limiter trip. Moisture ingress around underfloor heating manifolds is a known cause of connection problems.
- Faulty or failing PCB Rare
If the limiter and its wiring both check out correctly, the PCB itself may be misreading the external switching input or has developed a fault on that input channel. PCB failure is less common but worth investigating once other causes have been ruled out.
How to fix it
- Check your system pressure DIY safe
Look at the boiler's pressure gauge (or the digital display on newer models). It should read between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, top it up using the filling loop — a small braided hose connecting the cold mains to the heating circuit, usually under the boiler. Turn both valves open slowly until the gauge reaches 1.2–1.3 bar, then close both valves. Low pressure can restrict water flow and cause the temperature limiter to trip.
- Reset the boiler — once DIY safe
Press and hold the reset button (marked with a flame and a triangle on most Greenstar models) for around 3 seconds until the display changes. Allow the boiler a minute to attempt a restart. If it locks out again immediately, do not keep resetting — repeated resets without fixing the root cause can cause secondary damage to the PCB. One reset attempt is reasonable; more than two or three is not.
- Check whether an underfloor heating limiter has tripped DIY safe
If your home has underfloor heating, locate the manifold (often in an airing cupboard or a dedicated manifold box). Many underfloor heating systems have a separate high-limit thermostat that can be reset manually by pressing a small button on the device. Check whether this has popped out. If it has tripped, press it back in — but bear in mind an engineer still needs to find out why it overheated in the first place before you rely on the heating long-term.
- Do not attempt to access or test the external limiter wiring or PCB yourself Gas Safe engineer
Tracing the switching circuit, testing continuity on the limiter terminals, and inspecting PCB connector blocks requires disassembly of the boiler casing and working on live low-voltage circuits. This work must be carried out by a Gas Safe registered engineer. Incorrect interference can void your warranty and create new faults.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will use diagnostic equipment to confirm whether the limiter has genuinely tripped due to overheating, test the pump output and system flow rate, inspect wiring and connections on the external switching input, and replace the temperature limiter, pump, or PCB as needed. Always ask for a written quote before any parts are ordered. If the boiler is over 10–12 years old and the repair cost is significant, ask your engineer for an honest opinion on whether replacement makes more financial sense.
Parts you may need
- External temperature limiter / safety thermostat · from £35
- Circulation pump · from £85
- PCB (printed circuit board) · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the D3 232 fault?
It is tempting, but not a good idea. If the temperature limiter has tripped because of a genuine circulation or overheating problem, resetting without fixing that problem means the limiter will trip again. Repeated resets while the underlying fault persists put extra stress on the PCB and can turn a relatively inexpensive limiter or pump repair into a much more costly PCB replacement. One reset to see whether the boiler recovers is fine; if it locks out again, stop and call an engineer.
What is the external switching contact on a Worcester Bosch boiler?
It is a set of terminals on the PCB designed to accept a signal from an external safety device — typically a high-limit thermostat on an underfloor heating manifold, a cylinder thermostat, or a dedicated overheat limiter fitted to the boiler's flow pipe. When that device detects a temperature above its safe limit, it opens its contact (breaks the circuit), which tells the boiler to lock out. Cause code 232 points specifically to this external switching input being in an open state.
How much does it cost to fix a D3 232 fault on a Worcester Bosch boiler?
Most people pay somewhere between £120 and £350 all in, covering the engineer's call-out, labour, and parts. If the fix is simply resetting or replacing the external temperature limiter, costs are towards the lower end. A pump replacement sits in the middle of that range. If the PCB turns out to be at fault, the part alone can cost £200–£350, and total repair costs can reach £400–£550 — at which point it is worth getting a new boiler quote alongside the repair estimate, particularly if your boiler is more than 10 years old.
Could my underfloor heating system be causing the D3 232 fault?
Yes, this is a very common scenario. Underfloor heating systems are typically designed to run at lower flow temperatures than radiator circuits, so they have their own high-limit thermostat on the manifold. If the water feeding that manifold gets too hot — because the mixing valve is set incorrectly, the pump has slowed, or the thermostat has become oversensitive with age — the limiter trips and opens the switching contact back to the boiler, triggering D3 232. An engineer can check the underfloor manifold settings and the condition of the limiter as part of the diagnosis.