Explanation: The controller has detected an open circuit on a zone. 01 OPEN means an open circuit on zone 1, 02 OPEN means am open circuit on zone 2, and so on.
A load of less than 30 milliamps on a master valve circuit will create this fault message. Operation of the zone 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.
If you are receiving an open circuit fault on all master valves and zones, it's a good idea to check that the common wire is fully inserted into the common port inside the controller. If there are multiple connections to the common port, make sure that they are properly connected. If you are using stranded wire, rather than solid wire for your common, remove it and twist the strands so that they are all together. Use a small flat head screwdriver to press the orange tab down fully and insert the wire. Release the orange tab and give the wire a slight tug to ensure that it is secured into place.