Lennox Error Code 12: Indoor unit not detected (Furnace)
Lennox error code 12 on a furnace means: Indoor unit not detected. It is also called Lennox diagnostic code 12.
Homeowner? What Lennox code 12 means for you
Call a technician Your furnace is reporting: Indoor unit not detected. The furnace has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Safe things you can check yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Make sure the furnace front door or panel is closed properly; many furnaces will not run with the door switch open.
- Look at the air filter. If it is grey and clogged, replace it; a blocked filter causes many shutdowns.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the gas valve next to the furnace is open (handle in line with the pipe). Do not force it.
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Lennox error code 12 mean?
The thermostat cannot find a Lennox communicating indoor unit (furnace or air handler) on the system. S30/S40 thermostat alert code 12.
Applies to: Lennox communicating gas furnaces (e.g. SL280UHV/SL280UHNV, EL296UHV) in S30/S40 smart thermostat systems
Common causes of Lennox code 12
- No power to indoor unit
- Open or loose R, i+, i-, C wire at mag-mount/subbase, smart hub or indoor control
- EIM not configured as furnace or air handler (non-communicating indoor unit)
A technician checks these and 1 other possible causes on site before replacing any part.
Lennox models that use code 12
Show all 38 models
EL195UH040NE36BEL195UH060NE36BEL195UH080NE48CEL195UH100NE60CEL296UH045XV36BEL296UH070XV36BEL296UH090XV36CEL296UH090XV48CEL296UH090XV60CEL296UH110XV48CEL296UH110XV60CEL296UH135XV60DML180UH045E36AKML180UH045XE36AKML180UH070E36AKML180UH070E36BKML180UH070XE36AKML180UH070XE36BKML180UH090E48BKML180UH090E60CKML180UH090XE48BKML180UH110E60CKML180UH110XE60CKML180UH135E60DKSL280UH060NV36ASL280UH070V36ASL280UH080NV48BSL280UH080NV60CSL280UH090V36BSL280UH090V48BSL280UH090V60CSL280UH100NV60CSL280UH110V60CSL280UH135V60DSL297UH040NV36BSL297UH060NV36BSL297UH080NV48CSL297UH080NV60C
Other Lennox Furnace codes
All Lennox Furnace error codes →
Lennox code 12: homeowner questions
What does Lennox code 12 mean?
Indoor unit not detected. The thermostat cannot find a Lennox communicating indoor unit (furnace or air handler) on the system. S30/S40 thermostat alert code 12.
Is it safe to keep using my furnace with code 12?
It depends on the cause. The furnace has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Can I reset Lennox code 12 myself?
You can try one reset (power off for 30 seconds, then on). If Lennox code 12 comes back, the cause is still there and a licensed technician should check it.
Who can fix Lennox code 12 near me?
Aurora Star Tech Inc. repairs Lennox furnaces in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
Homeowner help: Furnace Not Turning On: What to Check · Furnace Blinking Light: What the Flash Codes Mean
Troubleshooting guides
- Blowing cold air All brands · Furnace
- No heat All brands · Furnace
- Short cycling All brands · Furnace
- Blower motor does not run - Lennox SL280UHV ECM checks Lennox · Furnace
- Clear stored error codes in the error recall menu Lennox · Furnace
- Display shows two horizontal bars (soft disable) - Lennox SL297NV Lennox · Furnace
- EL296UHV: normal gas heat sequence (what should happen after a call for heat) Lennox · Furnace
- EL296UHV: what the display and the diagnostic button are telling you Lennox · Furnace
- Furnace will not run (no heat) - Lennox EL195UHNE Lennox · Furnace
- Furnace will not run (no heat) - Lennox SL297NV Lennox · Furnace
- Furnace will not run - Lennox SL280UHV checklist Lennox · Furnace
- Indoor blower does not run - Lennox EL195UHNE Lennox · Furnace
- Indoor blower does not run - Lennox ML180UHE Lennox · Furnace
- Poor ignition or combustion resonance - Lennox EL195UHNE Lennox · Furnace
- Poor ignition or loud combustion resonance - Lennox SL280UHNV Lennox · Furnace
- Program the unit size code on a replacement furnace control Lennox · Furnace
- Recall or clear the last 10 error codes (red LED button) Lennox · Furnace
- Test or reset the Low GWP refrigerant detection board (Test button) Lennox · Furnace
- Two horizontal lines on display (soft disable) - Lennox communicating Lennox · Furnace
Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.9. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.