Explanation: The controller has detected an open circuit on a master valve (MV1 = master valve 1, MV2 = master valve 2).
A load of less than 30 milliamps on a master valve circuit will create this fault message. Operation of the master valve will continue normally, but the fault message will not be cleared unless done manually or the load exceeds 30 milliamps.
Resolution: Open circuits can be caused by wiring issues or by faulty solenoids.
Using an electrical multimeter, you can test the wiring using an electrical multimeter on the ohms (Ω) setting. [See here for a tutorial on testing ohms] Typically, the valve circuit tests at 20-60 ohms.
Start by testing the solenoid and then the zone wire. If the solenoid tests fine, but the zone wires show a reading greater than 60 ohms when connected to the solenoid, there is most likely a break in the zone wire.
If the solenoid tests higher than 60 ohms it will need to be replaced.