Overview
The D3 232 error on a Worcester Bosch GB162 indicates the external switching contact has opened. In practice this means the boiler has detected that an external control or limiter that should be providing a 'call for heat' or permit operation is in the open state. External switching contacts can be part of a room thermostat, programmer/timer, cylinder thermostat, frost thermostat, an external high-limit/temperature limiter, an interlock, a wired remote on/off switch, or a linked building management/energy management system. When that contact opens the boiler will stop or refuse to fire because it thinks no demand or a fault condition exists outside the boiler. Severity is typically low to moderate: it usually causes loss of heating and/or hot water but is not in itself an immediate safety risk unless it is masking another problem that caused an external limiter to trip (for example overheating). In most cases the cause is an external control, wiring, or connector issue rather than an internal gas fault. Some basic checks can be done by a competent homeowner (status of timers, thermostats, fuses, and reset), but diagnosing wiring, measuring control voltages, replacing limiters or opening the boiler requires a qualified Gas Safe engineer or electrician. If there is any smell of gas, visible damage, repeated lockouts, or you are unsure, do not attempt internal repairs and call a professional immediately.
Possible Cause: The external switching contact has opened.
Troubleshooting Steps
Safety precautions:
- If you smell gas evacuate the property and call the gas emergency services—do not operate switches, buttons or appliances.
- Isolate the electrical supply to the boiler before opening covers or touching internal wiring. If you are not competent with electrical work do not open the boiler.
- Do not bypass or defeat any safety or limiter devices. If an external limiter has tripped it is doing so to protect the system.
Initial homeowner checks you can safely do:
1. Check that the boiler mains switch is ON and that there is power to the boiler (LEDs on, display active). Check your home fusebox for a tripped fuse or RCD.
2. Check your room thermostat/programmer: set the programmer to a heating state and raise the thermostat target temperature above room temp to force a call for heat. If you have a wireless thermostat check batteries and pairing.
3. Check any external controls tied to the boiler: cylinder thermostat, frost stat, external timers, remote on/off switches, or any building management interlocks. Ensure they are set to call for heat or are not in a manual OFF position.
4. If applicable, check that any zone valves are in the correct position (motorised valves typically show a change in position when calling for heat) and that radiators/valves are open.
5. Try a reset of the boiler only if there is no gas smell and no visible fault: press the boiler reset button as described in the boiler manual (typically hold for 3–5 seconds) and wait 2–3 minutes for it to attempt a restart. If the boiler returns to normal, monitor whether the issue repeats.
If the fault persists or returns after reset (diagnostic steps that may require a qualified person):
1. Visually inspect accessible wiring and connectors between external controls and the boiler for loose terminals, corrosion, rodent damage, or water ingress. Do not touch live terminals with the power on.
2. Identify the external switching terminals on the boiler using the manual/wiring diagram. Many systems use either mains switching or a low-voltage (e.g. 24 V) signal. If you are competent with electrical measurement and it is safe to do so, measure whether the external contact is opening when the control should be calling for heat. If you are not competent, stop here and call a professional.
3. If an external thermostat or programmer is suspected, temporarily link the appropriate switching terminals on the boiler to simulate a call for heat (only if you know the correct terminals and are confident and permitted to do so). If the boiler fires when the terminals are bridged, the fault is with the external control or its wiring; do not leave a permanent bridge in place—this is a diagnostic check only.
4. If an external temperature limiter (high-limit or safety limiter) has tripped, check whether that device has a manual reset (some do). If it has, reset it following the manufacturer’s guidance. If it trips again immediately or soon after, do not reset repeatedly—this indicates a deeper fault such as overheating, blocked flow, pump failure, closed valves, or sensor failure.
5. Check system water pressure on the boiler gauge. Very low pressure (< 1 bar) or a pump not running can cause abnormal behaviour and may indirectly trigger limiters. Re-pressurise the system only using the correct filling loop procedure in the manual.
6. Observe for related symptoms: repeated opening of the external limiter during heating can point to overheating of the primary circuit, flow sensor or pump issues, blocked heat exchanger or diverter valve faults. These must be investigated by an engineer.
When to call a professional and what to tell them:
- Call a Gas Safe registered engineer if: you are not confident performing electrical checks, the boiler will not reset, the fault returns repeatedly, an external limiter repeatedly trips, you suspect wiring or PCB faults, or there are any signs of overheating or internal component failure.
- Provide the engineer with the boiler model (GB162), the exact fault code D3 232, and a clear description of the checks you have already carried out (power on, reset attempted, thermostat/programmer status, pressure reading, any bridging test results). This speeds diagnosis.
Final notes and warnings:
- Do not attempt to replace gas or sealed combustion parts yourself. Only authorised Gas Safe engineers should work on gas components and internal boiler parts.
- Do not bypass safety limiters or permanently bridge control terminals to force operation. These devices protect people and equipment.
- If the boiler is under warranty or covered by a service contract, contact your installer or Worcester Bosch technical support for guidance before commissioning significant repairs.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.