Honeywell Home Security F21 Fix: Zone 21 Fault Quick Guide

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What This Error Means

On Honeywell home alarm panels, F21 basically means: Zone 21 Fault.

The panel thinks the sensor on zone 21 (usually a door, window, or motion) is open, tampered, or not talking to the system, so it refuses to arm that zone.

Official Fix

Do it the clean, manual-friendly way:

  • Figure out what zone 21 actually is. Look at the keypad text next to F21 (often a name like “Back Door” or “Hall Motion”) or your zone list from the installer. You are hunting for the real-world thing called “Zone 21”.
  • Make sure the thing is actually closed. If it’s a door or window, shut it fully and latch it. Watch the keypad; if F21 clears, you were just dealing with something left open.
  • Check the door/window contact alignment. For magnet contacts, the two pieces should be almost touching, gap under 1/4 inch, and lined up straight. If the magnet is crooked or pulled away, reposition or re-mount it so the pieces line up again.
  • Inspect the sensor for damage or tamper. Cracked housings, loose covers, or a cover that will not snap tight can hold the tamper switch open and cause F21. Reseat the cover firmly.
  • Wireless sensor? Swap the battery. Pop the cover, note battery type, and replace with a fresh, name-brand cell. Wait 30–60 seconds. Close the cover tight so the tamper switch is pressed. Watch the keypad to see if F21 disappears.
  • Wired contact? Check the wiring (only if you are comfortable). Put the system in disarm, then remove power (unplug transformer and disconnect backup battery). Check that both wires are still landed under the sensor screws and not broken or corroded. Tighten loose screws, re-terminate broken wires, then restore power.
  • Motion or glassbreak on zone 21? Make sure it still has power (LED flashes during walk test), is firmly mounted, and its cover is fully seated. Replace its battery if it is a wireless unit.
  • If the device is missing altogether, do not guess. Sometimes zone 21 is a sensor that was removed during a remodel. In that case the panel will sit in permanent F21 until an installer deletes or reprograms that zone. Call your alarm company or a local tech to clean up the programming.

To clear the F21 message once the hardware is good:

  • Make sure the system is disarmed.
  • Close and secure the zone 21 device.
  • Arm and then disarm once (or follow your panel’s manual steps to clear faults). If the zone is really fixed, F21 will drop off the display.

If F21 will not clear after all of that, the zone 21 sensor is probably failing internally and needs to be replaced or removed from programming.

The Technician’s Trick

What techs actually do in the field when F21 is wasting time and the hardware is flaky:

  • Bypass zone 21 so you can still arm. On most Honeywell keypads you can use the Bypass button, then pick zone 21, to arm the rest of the system while you wait on parts or a visit. The exact button combo varies by panel, so watch the screen prompts, but the idea is simple: mark zone 21 as bypassed, arm, then fix it properly later.
  • Hard reboot a stubborn panel. If the sensor is fine but F21 is stuck, do a full power cycle:
    • Call your monitoring company and put the system on test so trips do not dispatch police.
    • Unplug the AC transformer.
    • Open the main panel can and disconnect the backup battery.
    • Wait 30 seconds.
    • Reconnect the battery, then plug AC back in and let the panel fully boot.
    If F21 was just a logic hiccup, it will clear after reboot.
  • Fix a touchy wireless tamper switch. If a wireless sensor on zone 21 only faults when the door slams or when you bump it, the tamper spring may be loose. A tech will slightly bend the metal spring up or add a thin cardboard shim so the cover presses it firmly. That stops random F21 tamper faults without replacing the whole sensor.

These tricks trade a bit of security for usability. Use them to get through the night, then schedule a proper repair.

Is It Worth Fixing? (The Financial Verdict)

  • Fix: The panel is under 10–12 years old, F21 is tied to a single sensor, and you just need batteries, a new contact, or a quick wiring repair.
  • Debatable: You have multiple zones randomly faulting (not just 21), the keypad is fading or missing pixels, and you are already paying for monitoring that could support a newer panel.
  • Replace: The system is 15+ years old, parts are hard to find, you want app control and modern features, or a tech quotes more than the cost of a basic new Honeywell/Resideo or DIY smart alarm kit.

Parts You Might Need

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See also

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