AC loss on an ADT system means the security panel isn't getting enough alternating current (AC) power from the wall outlet, so it switches to battery power and may disable some features until the issue's fixed.
Quick Fix Summary
Check if the outlet works, look at the transformer plug and connections, then power-cycle the system before assuming something's broken.
Start simple: plug in a phone charger or lamp to test that outlet. No power? Reset the breaker or try another outlet. Next, unplug the transformer from both ends, wait 10 seconds, then plug it back in. If the AC loss light goes away, problem solved. If not, keep reading.
What's Happening
AC loss triggers when the ADT panel gets less than 16–18 volts AC from the wall transformer, forcing it to run on battery power and showing an error.
Usually this comes from a tripped breaker, dead outlet, loose transformer plug, or a transformer on its last legs. According to the ADT support page, you'll see “AC Loss” or “Check AC” on the panel. Let it go too long and your battery drains faster—eventually the alarm stops reporting entirely.
Step-by-Step Solution
Run through four quick checks: test the outlet, inspect the transformer, measure voltage, then reset the system to get AC power back.
- Plug a lamp or phone into the outlet to confirm it's working.
- Unplug the transformer from both the wall and the panel; look for burn marks or bent prongs on both ends.
- Use a multimeter to check the transformer's output—you want around 16–18 volts AC. Anything lower means the transformer's weak.
- Power-cycle: unplug the transformer, pull the backup battery for 30 seconds, then reconnect everything and power the system back on.
Still seeing the error? Grab the model-specific reset guide from the ADT support site or call ADT directly.
If This Didn't Work
Look for damaged wires, replace the transformer and battery, and check the event log for codes like “Telco Fault” or “Check 94” before calling a pro.
Check for gnawed wires, corroded transformer terminals, or a battery that won't hold a charge anymore. Swap in a new transformer if the voltage is low or the casing is cracked. The event log often shows whether it's really an AC problem or something else—saving you from swapping parts you don't need. If nothing works, book a certified ADT technician.
Prevention Tips
Keep your system healthy by testing monthly, checking connections every six months, and replacing the battery and transformer on schedule to avoid future AC loss headaches.
| Component |
Maintenance Frequency |
Lifespan |
| Outlet |
Every 6 months |
Ongoing |
| Transformer |
Every 5–7 years |
5–7 years |
| Backup Battery |
Every 3–5 years |
3–5 years |
Keep the panel away from heat and moisture, and label the circuit breaker so you can flip it back on quickly after power outages. The National Electrical Manufacturers Association backs these simple checks to keep security systems running smoothly all year.
Edited and fact-checked by the TechFactsHub editorial team.
