Watchguard appears across Lennox’s documentation. The older two-LED SureLight-era instructions describe it in one sentence: Watchguard — burners failed to ignite, limit open longer than 3 minutes, or flame sense lost 5 times in one heating cycle, shown as both LEDs flashing slowly, alternating. The newer -02 integrated control gives Watchguard its own numbered codes:
| Code | Watchguard meaning (Lennox -02 control table) |
|---|---|
| 5 flashes | Watchguard — burner failed to light; maximum number of retries or recycles exceeded. |
| 7 flashes | Primary or secondary limit open — or Watchguard mode, when a limit stays open longer than 3 minutes. |
| 10 flashes | Watchguard — pressure switch opened 5 times during one heat demand. |
Source: Lennox EL180 Unit Information, Table 4 (-02 Integrated Control), and the Lennox G50UH installation instructions’ two-LED diagnostic list. Full tables on the Lennox codes page.
The three doors into Watchguard
1. It never lit
Retries exhausted with no proven flame. Gas supply at the furnace, the igniter, and the flame sensor are the technician’s first three checks — yours are the thermostat call, the filter, and confirming the gas shut-off at the furnace is open.
2. A limit stayed open past 3 minutes
The overheat limit opened — classically airflow: a matted filter, closed registers, blocked returns — and did not close again inside 3 minutes. This is the one Watchguard door with a genuinely homeowner-sized cause. Change the filter and open the registers before you pay anyone.
3. Flame sense lost 5 times
The furnace lights, then the flame signal drops, five times in one call for heat. That is a sensing problem — the flame sensor and its signal — and it belongs to a technician with a meter.
Getting out of Watchguard
Watchguard releases when the fault clears and the control’s timer runs out, or when power is cycled after the cause is fixed. Cycling power on a furnace that still cannot light just re-enters the same retries and lands back in Watchguard — the mode is doing its job. Fix the door it came in by, then reset once.
The two rules that outrank everything on this page.
1. Smell gas? Stop reading. Leave the house and call your gas utility or 911 from outside. No code is worth diagnosing through a gas smell.
2. Never bypass a safety switch — no jumping a pressure switch, no taping a limit, no propping a rollout. Those switches are the only thing between a furnace fault and a fire or carbon-monoxide problem. This library explains codes; it does not teach repairs inside the cabinet.
Related: the full Lennox flash-code tables (both control generations), and lockout behavior across all six brands.
One honesty note before the tables: codes change between control boards, sometimes inside the same brand and the same decade. Every table below names the board and the manual it was transcribed from. The chart glued inside your own blower door is the final word for your furnace — if it disagrees with this page, believe the door.