What troubleshooting steps can I take before calling a professional for Baxi error 125?

First, check the pressure gauge on your boiler; if it's below 1.0 bar, repressurize the system using the filling loop as described in your manual. Second, check your radiators and pipework for any visible leaks. Third, ensure all radiator bleed valves are closed. If you've recently bled radiators, repressurizing is expected. If the error returns quickly after repressurizing, or you cannot identify a leak, then a professional is needed.