I fitted a new oil pressure sensor over the weekend and the fault disappeared.................... for a day
Now I know that the oil pump is fine, because once the warning message has gone out after the initial start, it is fine and you can drive all day with no messages. This problem is exclusively on start up only, but not every start up and it happens with both hot and cold engine conditions.
The engine oil pressure does seem to take a while to build up, there are no strange noises although I have noticed that the hydrulic lifters are a tad noisier after a cold start and soon quieten down.
I'm now suspecting a problem with the oil pressure relieve valve, sticking open, thus pressure being lost. And if it is sticking then that could explain why it is randomly happening.
Another thought is the non-return valve, allowing oil to drain from the pump, I know that the oil filter is in effect fitted with one of these, but is there one in the pump as well?
And lastly how accessable is the pump etc?