Error S.96

Safety precautions:

- Do not attempt gas or detailed electrical work unless you are a Gas Safe registered engineer. Working inside the boiler voids warranties and can be dangerous.

- Before touching any wiring or opening the boiler cover, switch the boiler OFF at the room thermostat and at the mains isolator, and isolate the electrical supply to the boiler. Allow the boiler to cool so you won’t burn yourself on hot pipes.

- If you detect a gas smell or a water leak you should isolate the gas (if you know how) and water supply and call an engineer immediately. Do not attempt to light or test the boiler in that condition.

Initial checks a homeowner can do (safe, non-invasive):

1. Note the exact code shown and any other codes or messages. Write down what you see and the circumstances the fault appeared (e.g. after a power cut, after service, after bleeding radiators).

2. Try a simple reset: switch the boiler off for one minute at the mains, then switch back on and see if the S.96 reappears. A single transient fault can sometimes clear, but repeat or persistent faults indicate a real issue.

3. Check the boiler water pressure on the front panel. If pressure is very low (<1.0 bar typically) this can cause circulation problems that may affect temperature readings. Re-pressurise the system per the boiler manual if you know how; if unsure, call an engineer.

4. Visually inspect any external sensor wiring or visible connectors without opening the boiler: look for disconnected plugs, damaged insulation or water ingress on cable runs you can access externally. Do not force connectors or probe inside the boiler while it is live.

5. Run the central heating briefly (if safe) and carefully feel the flow and return pipes at the boiler (or the first radiator): the flow pipe should be noticeably hotter than the return. If both pipes are cold or nearly the same temperature while the pump is running, this points to a circulation issue rather than just the sensor.

Specific diagnostic and fix steps (for the engineer / recommended actions to request):

1. If the basic homeowner checks have not cleared the fault, arrange for a Gas Safe engineer. Ask them to bring a multimeter and the correct replacement NTC sensor for your model.

2. The engineer will isolate power and open the boiler, inspect and reseat the return sensor connector, and inspect the wiring harness for shorts, breaks or water damage.

3. The engineer should measure the sensor resistance at known temperatures and compare to the specification (NTC characteristic) to confirm open-circuit or short-circuit or out-of-range readings. They will also check continuity to the PCB and for any short to earth.

4. If wiring and connectors are good but the sensor shows incorrect resistance, the return thermistor should be replaced and the new sensor retested.

5. If the sensor and wiring are correct but the PCB shows implausible readings or faults, the engineer will test the PCB inputs and replace the control board or repair wiring as required.

6. If the fault is related to circulation (pump failure, blocked pipe, airlock, diverter valve fault) the engineer should diagnose and repair the hydraulic cause, because persistent circulation issues can cause false temperature sensor errors.

7. After any repair the engineer will clear fault codes, run the boiler through operating cycles, and verify flow and return temperatures are sensible and that no other faults appear.

Final note: S.96 is a sensor/electrical fault that is commonly fixed by replacing a faulty thermistor or repairing wiring, but diagnosing it correctly requires electrical testing and possibly dismantling the boiler. For safety and compliance you should call a Gas Safe registered engineer or contact Vaillant customer service (Vaillant helpline as applicable) to arrange diagnosis and repair. Do not attempt internal repairs or component swaps yourself unless you are qualified.