What should I do if repressurising the boiler doesn't resolve error 101?

If repressurising doesn't fix the issue, it suggests a leak is present. Carefully inspect all radiators for leaks, check pipework for dampness, and examine the boiler itself for any signs of water escaping. If you can't find the leak, or if it's internal to the boiler, it's best to call a qualified heating engineer to diagnose and repair the problem.