Oil boiler locking out

[wishuwerehere]

We have an Potterton oil boiler with a Selectos burner, it was serviced a few months ago and has worked ok until last week. Now it locks out frequently. We have had it looked at, and the photocell and control box were changed but it continues to lockout. The oilflow to the pump is gravity fed, and lots of it. It might go for 10 minutes or 2 hours. There is nothing in the manual that deals with this problem. What else could the problem be?

 

[MANDATE]

A few years ago I had a similar problem, but my oil flow is not gravity fed.
It coincided with the removal of a steel tank on a stillage being replaced with a plastic tank at ground level.
Everything seemed OK while the new tank was full, but when it got low I had many lockouts. After purchasing a oil pressure gauge I found the output pressure was about 18 lbs. sq. in. and could not be increased to the spec requirement of 100 lbs./ sq. in. This was due to a worn out pump which had been in use for about 15 years.

 

[wishuwerehere]

Thanks Mandate, checked the pressure, it was 120psi so not that. Have a suspicion it may be the pump solenoid going open circuit when it gets hot. Tried a new one, ok so far.