I have a 2011 Ford Fiesta 1.6 TDCI . On the first start up in the morning the oil pressure light will come on for a split second with a chime then go off this only happens in the morning on the first start and never comes on again throughout the whole day. This happened approximately 6 months ago. We done a oil pressure check with test gauge by removing the switch and found the oil pressure was fine. So put it down to the oil pressure switch which we replaced and it resolved the fault. However 6 months later it has started doing the same thing again. So we thought the switch might be faulty and got another one from Fords but its still doing the same thing. Ideas what it might be? 

Could be the oil pick up pipe is blocked or has debris on it so it, happened to my old astra j only done it in the morning on first started up and the light was on for 5 seconds, mine was the oil pick up seal on that so was a sump off job and clean the sump pipe and fit a new pick up seal. So could be the same as the fiesta, sounds like the same symptoms anyway.

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My 2011 mondeo 1.6 tdci has just done this for the first time this morning ( no chime but oil light stayed on for a second after starting) due a service soon (8,000m). I suspect this could be a blocked oil pick up strainer issue?