Lennox is the brand where asking "which board?" matters most after Goodman. The older SureLight control counts simple flashes 1-9. The newer -02 integrated control counts to 13, flashes them fast (half a second on, half a second off), stores its last 10 faults behind a recall button — and assigns completely different meanings to the low numbers. A Lennox "3 flashes" is a pressure-switch code on SureLight and a failed-ignition code on the -02 control.
And if your furnace or its paperwork says Watchguard, that is Lennox’s lockout mode — it has its own page here, because it is the word Lennox owners actually search at midnight.
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.
Which chart is yours?
Single LED, codes that stop at 9, and a furnace old enough to have a SureLight board: first table. A board with a recall button and codes that run past 10 — or any EL180-series unit: second table. The -02 control’s heartbeat (one second on, one second off) is its normal powered state, listed at the top of its table.
SureLight single-stage control — flash codes
Lennox furnaces on the single-stage SureLight ignition control (the table ships with Lennox’s SureLight replacement-control kit 19V36 and the furnaces that use that control). The control stores its last 5 fault codes.
| What you see | What the manual says it means |
|---|---|
| LED off | No power to the control, or a control hardware fault — One of only two brands here where a dark board is itself a listed diagnosis. |
| LED on | Normal operation — Steady light, happy furnace. |
| 1 flash | Flame present with the gas valve de-energized — The sensor sees flame the board did not order. Gas code — furnace off. |
| 2 flashes | Pressure switch closed with the combustion air inducer de-energized — Stuck closed: the switch should be open while the inducer rests, and it is not. |
| 3 flashes | Pressure switch open with the combustion air inducer energized — Stuck open: the inducer is pulling and the switch will not close. Vent, drain, hose, or the switch itself. |
| 4 flashes | Primary limit switch open — The main overheat limit tripped. Airflow homework first. |
| 5 flashes | Rollout switch open — A rollout switch opening means flame went somewhere it should not. It is a manual-reset switch, and it should not just be reset and forgotten. |
| 6 flashes | Pressure switch cycle lockout — The switch kept dropping out mid-cycle until the control locked the cycle down. |
| 7 flashes | Lockout — burners fail to light — Retries exhausted with no ignition. Gas supply, igniter, or sensor — in the order a tech will check them. |
| 8 flashes | Lockout — burners lost flame too many times — It lights, then loses flame, repeatedly. A flame-signal problem until proven otherwise. |
| 9 flashes | Line voltage polarity incorrect — Hot and neutral are swapped. Electrician territory. |
Source: Lennox SureLight Ignition Control Kit 19V36, Table 4 - Diagnostic Codes. Read the document (fetched 2026-10-30).
Newer integrated control (-02) — numbered flash codes
The -02 integrated control in Lennox’s EL180 unit information (2017). Errors flash rapidly — count the flashes, then a 3-second pause, then it repeats. The board remembers its last 10 codes.
| What you see | What the manual says it means |
|---|---|
| Heartbeat (slow flash) | Control powered — normal — One second on, one second off, repeating: the control is alive and sees no errors. It shows in every mode, so a heartbeat plus no heat means look at the thermostat call, not the board. |
| 1 flash | Reverse line voltage polarity — Swapped hot and neutral. |
| 2 flashes | Improper earth ground — The ground this control depends on for flame sensing is not right. |
| 3 flashes | Burner failed to light, or lost flame during the heat demand — The workhorse code on this control: ignition or flame proof failed mid-demand. |
| 4 flashes | Low flame signal — check the flame sensor — Lennox’s own table entry names the sensor. A tech cleans it and measures the signal. |
| 5 flashes | Watchguard — burner failed to light; maximum retries or recycles exceeded — Watchguard is Lennox’s name for its protective lockout mode. It is the board saying: I have tried enough times; a human should look now. |
| 6 flashes | Not used — Lennox’s table literally lists code 6 as not used on this control. If you counted 6, recount — you probably merged two stored codes. |
| 7 flashes | Primary or secondary limit open — or Watchguard (limit open longer than 3 minutes) — One code covers both a live open limit and the lockout that follows if it stays open past 3 minutes. |
| 8 flashes | Rollout switch open — Flame-safety switch. Off, and call. |
| 9 flashes | Pressure switch failed to close, or opened during the heat demand — Both failure directions in one code: never closed at start-up, or dropped out mid-run. |
| 10 flashes | Watchguard — pressure switch opened 5 times during one heat demand — The five-strikes version of code 9. |
| 11 flashes | Pressure switch stuck closed before the inducer started — The switch was already closed when the cycle began — the board catches this before anything else runs. |
| 12 flashes | Flame sensed without the gas valve energized — Flame signal with no order for flame. Furnace off, call in. |
| 13 flashes | Low line voltage — The supply voltage is sagging below what the control accepts. Utility or wiring. |
Source: Lennox EL180 Unit Information (09/2017), Table 4 - -02 Integrated Control. Read the document (fetched 2026-10-30).
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.
Questions
What is Watchguard on a Lennox furnace?
Lennox’s name for its protective lockout. It triggers when burners fail to ignite through all retries, when a limit stays open longer than 3 minutes, or when flame sense is lost 5 times in one heating cycle. Codes 5, 7 and 10 on the -02 control are Watchguard codes.
How do I read stored Lennox codes?
On the -02 control, press and release the recall button: the board flashes stored codes most-recent-first, with the LED off for 3 seconds between them. It remembers the last 10, even through a power cut. Holding the button more than 5 seconds clears them — photograph them first.
Where these meanings come from
Transcribed 2026-10-30 from the manufacturers’ own documents — not from other blogs:
- Lennox SureLight Ignition Control Kit 19V36, Table 4 - Diagnostic Codes — read it
- Lennox EL180 Unit Information (09/2017), Table 4 - -02 Integrated Control — read it