X-Core - "NO AC" on Display The "NO AC" display is a diagnostic message telling you that the controller has stopped receiving AC power. The controller is running on the power supplied by the backup battery. Power Source Check the power source by plugging a lamp or radio into the wall outlet to verify there is power at the outlet. If your controller is not the indoor model with square plug, then the power source is hardwired. If you do not have power to the outlet, check all wall switches, timers, or power strips that may be connected to the outlet. You may need an electrician to check the GFI, circuit breaker, and electrical outlet if you do not have power. Controller Power When power is verified coming into the controller, the next step is to disconnect the backup battery. Steps to remove the battery: Open wire compartment cover by pushing straight down. Remove the phillips screw from the battery holder. Slide the battery holder down to access the battery. If the display goes blank after removing the battery, the transformer could be damaged and may need to be replaced. If the display continues with the "NO AC" message, then it needs a hard reset. Transformer Testing If you a have a multi-meter, you could also try testing for 24VAC on the two AC terminals (Yellow wires) inside the wiring compartment. Normal voltage on these terminals is between 24- 28 volts AC. If there is no voltage on the AC terminals, the transformer needs to be replaced. Indoor Controller Transformer - 526500 Outdoor Controller Transformer - 468000