If the oil pressure warning lamp is on or flashing, this indicates insufficient oil pressure in the system. Remove and check the valve.
Unscrew the stopper (1).
Remove the spring (2) and piston (3).
Check piston free play, clean if necessary replace piston. The piston must show no signs of wear.
Insert the piston and spring, place a new aluminum or copper o-ring under the plug and tighten.