Rheem Furnace Code 12
Warning
Low flame sense. The flame current the control reads from the flame sense rod is weak or marginal. The furnace keeps running normally, but if the signal falls further the code escalates to 13 (flame lost) or 11 (failed ignition).
Manual wording: “LOW FLAME SENSE ... The flame sense current from the flame sense rod at the furnace control (or I.F.C.) is weak or marginal at best.”
Documented for: (-)97V, (-)96V, (-)801S/(-)801P
Likely causes, most common first
- The flame sense rod needs cleaning, is not properly connected, or its wiring to the control is shorted or open
- An improperly mounted or poorly grounded flame sensor
What you can safely check yourself
- Nothing homeowner-serviceable - the flame sensor is inside the burner compartment. Book service before it turns into a no-heat call.
When to call a technician
Code 12 is an early warning. Have a technician clean or replace the flame sense rod and check the furnace ground before it becomes a lockout.
A Rheem furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Sources
- Installation Instructions for Upflow Condensing Modulating, Communicating Gas Furnaces - (-)97V Series (92-24161-126-04) (Rheem Manufacturing Company, R97V series modulating condensing gas furnace (and R98V); TABLE 18 normal operation codes, TABLE 19 fault codes, pages 94-104) · checked 2026-08-25
- Installation Instructions for 4 Position Condensing Two-Stage, Communicating Gas Furnaces - (-)96V Series (92-24161-124-08) (Rheem Manufacturing Company, R96V series and (-)(-)96MDV two-stage condensing gas furnaces; TABLE 19 normal operation codes, TABLE 20 fault codes, pages 111-120) · checked 2026-08-25
- Installation Instructions for Downflow Single Stage Gas Furnaces - (-)801S / (-)801P Downflow Series (92-24161-143-03) (Rheem Manufacturing Company, R801S, R801P, (-)(-)80DSS, (-)(-)80DSP 80% single-stage gas furnaces; DIAGNOSTICS AND FAULT CODES, page 34) · checked 2026-08-25