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Lennox Merit ML180UH Furnace Codes
| Code | Meaning | What to check |
|---|---|---|
| Off | No power to the control, or control hardware fault detected. (Off is also shown during normal operation.) | Confirm 115 VAC and 24 VAC at the control and the door switch. |
| 1 flash | Flame present with gas valve de-energized. | Gas valve leaking through or a shorted flame sense circuit. How to fix → |
| 2 flashes | Pressure switch closed with combustion air inducer de-energized. | Stuck or jumpered pressure switch. How to fix → |
| 3 flashes | Pressure switch open with combustion air inducer energized. | Blocked vent or intake, cracked or plugged hose, weak inducer, wet switch. How to fix → |
| 4 flashes | Primary limit switch open. | Airflow: filter, blower, closed registers, dirty coil, high temperature rise. How to fix → |
| 5 flashes | Rollout switch open. | Flame rolling out: blocked heat exchanger or flue. Find the cause before resetting. How to fix → |
| 6 flashes | Pressure switch cycle lockout. | Pressure switch opening repeatedly during the cycle. How to fix → |
| 7 flashes | Lockout, burners fail to light. | Gas supply, igniter, gas valve, flame sensor. How to fix → |
| 8 flashes | Lockout, burners lost flame too many times. | Flame sensor, gas pressure, grounding, flame carryover. How to fix → |
| 9 flashes | Line voltage polarity incorrect. | Hot and neutral reversed, or poor ground. How to fix → |
How to read the codes
- With power on, find the LED on the integrated control and count the flashes between pauses.
- Read the code before cycling power or opening the blower door, since a reset can clear it.
Codes point to a circuit, not a part. Test before replacing anything, and never jumper a limit, rollout or pressure switch to keep a furnace running. Gas and line-voltage work is for trained technicians.
Source: Lennox ML180UH Installation Instructions, 507323-01 (07/2018), Integrated Control Diagnostic Codes. The "meaning" column follows the manufacturer's wording; "what to check" is our field summary. Always go by the label on the furnace you're working on.
Related
- How to read furnace LED flash codes
- Furnace pressure switch troubleshooting
- Flame sensor microamps and cleaning
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