Ian wrote:

I've got an Oase 1700 pump in the pond.
When I turn it on, the impeller won't start, it just vibrates a little.
If I take off the inlet cap and nudge the impeller, it starts.
I then put the inlet cap back on, and it'll run indefinitely.#

If it has a start capacitor, that could have failed, whether that would
be accessible to replace it on a submersible pump is another matter ...