Overview
Error 260 on a Worcester Bosch GB162 means the boiler's flow temperature sensor did not detect a rise in the flow temperature after the burner tried to start. In plain terms the boiler fired (or attempted to fire) but the controller did not see the expected increase in the water temperature leaving the boiler. The controller interprets that as a sensor or circulation problem and will either generate the 260 fault or lock out if the condition persists. Common reasons for this are a failed or disconnected NTC flow temperature sensor, a broken or shorted sensor lead or connector, a pump or circulation failure so the heated water is not moving past the sensor, an airlock in the system, a closed valve, a blocked heat exchanger or a faulty control/burner electronics that misread the signal. Severity is moderate: it usually results in loss of heating and/or hot water and can lead to repeated lockouts. Some basic checks can be done by a competent homeowner, but sensor replacement, wiring repairs, pump replacement or any work involving the gas or combustion chamber must be done by a Gas Safe registered engineer.
Possible Cause: The flow temperature sensor is not detecting a rise in temperature following a burner start.
Troubleshooting Steps
Safety first: do not attempt any work on gas components, the burner or live electronics unless you are a qualified Gas Safe engineer. Isolate electrical power to the boiler before removing panels or touching wiring. Never attempt to modify or bypass safety devices. If you smell gas, evacuate and call the gas emergency number immediately.
Initial homeowner checks you can safely do:
1) Reset the boiler: press the reset as the manual describes and see if the fault clears. If it clears but returns, note when it happens.
2) Check the system pressure on the boiler gauge. If pressure is very low (typically below about 1.0 bar) top up the system to the recommended pressure (check the manual) and re-test. Low pressure can prevent circulation.
3) Check thermostat and programmer demand: ensure room stat or programmer is calling for heat. For testing, set the boiler to demand central heating and observe.
4) Listen and feel: when the boiler tries to start listen for the fan/pump running. Carefully feel (without touching any hot metal or terminals) the flow pipe coming out of the boiler after a burner start attempt. If the burner fires and the flow pipe warms within a minute or two, the sensor is likely working. If the burner fires but the flow pipe remains cold, there is likely a circulation or heat generation issue.
5) Bleed radiators: if radiators are cold at the top, there may be airlocking preventing flow. Bleed a few radiators and re-check.
Specific diagnostic steps (safe for a competent DIYer; stop and call a pro if unsure):
1) Record exactly when the 260 appears and any other codes shown concurrently. Multiple codes can point to wiring or PCB issues.
2) Observe sequence: does the boiler attempt to ignite? If the burner does not light at all then the fault may be mis-identified; check for ignition faults or Cx codes instead and call an engineer.
3) Confirm pump operation: when CH demand starts you should hear or feel the pump running. If the pump is silent, stuck or not circulating, the flow sensor will not register a temperature rise. Pump problems, electrical supply to pump or pump jam are common causes and require a qualified engineer to remove/replace the pump safely.
4) Visual check of external wiring: with the boiler isolated from mains power and the outer panel removed only if you are competent, check for obvious loose sensor connectors, damaged insulation or disconnected plugs at the flow sensor connector and the control board. Reseating a loose plug can restore function. If you are not comfortable removing panels, stop and call an engineer.
5) Check for blocked condensate/heat exchanger or closed isolation valves: visible closed piping valves on the flow or return can prevent circulation. If you find closed valves and can safely open them, do so and test again. Do not force valves or tamper with boiler internals.
6) If the burner fires, pump runs and the flow pipe still does not warm but the flow sensor reads nothing, the flow sensor is likely faulty (open/short or failed NTC). Replacing the sensor is a straightforward engineer task but should not be attempted by non-qualified persons because of gas and electrical safety requirements.
7) If wiring and sensor look intact but faults persist, the control electronics (PCB or burner control unit) can be at fault; this requires diagnostic tools and must be handled by a Gas Safe engineer.
When to call a professional: call a Gas Safe registered engineer if the reset does not clear the fault, if you find the pump is not running, if you are not confident removing panels or checking connectors, or if any wiring, sensor replacement or burner-related work is indicated. Sensor replacement, pump replacement, heat-exchanger or PCB work are not DIY jobs. Provide the engineer with the fault code 260, the behavior you observed (burner/no burner, pump/no pump, flow pipe warm/cold) and any other displayed codes to speed diagnosis.
Final note: do not attempt to bypass temperature sensors or safety interlocks. Those devices protect against unsafe operation and bypassing them can cause dangerous conditions. If in doubt, stop and get a Gas Safe engineer to inspect and repair the boiler.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.