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Rheem 12: Low flame sense: flame sense current is weak or marginal. Furnace keeps running

Ref. RHEEM-12 · From the Rheem furnace manual · Updated 2026-10-23

Status code12Rheem
What it means

Low flame sense: flame sense current is weak or marginal. Furnace keeps running.

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How it shows: Dual 7-segment display: 12

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§1Likely causes

  • Flame sense rod needs cleaning or is not properly connected
  • Wiring between rod and control shorted or open
  • Improperly mounted or poorly grounded flame sensor

§2Safe DIY steps

  1. Technicianclean or replace the flame sense rod and check connections
  2. Technicianreinstall/replace sensor and confirm furnace ground
Safety firstIf you smell gas, do not touch any electrical switch or use a phone in the building; leave immediately and call your gas supplier from outside. Turn off electrical power at the furnace switch or breaker before opening any panel, and never bypass or jumper a limit, rollout or pressure switch. Gas valve, burner, venting, wiring and control-board work should be done by a licensed HVAC technician.

§3When to call a pro

Call a licensed HVAC technician: to service the sensor before it causes lockouts.

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§4Other Rheem codes

10Rheem

One-hour lockout, caused by repeated failed ignition (11), water sensed (59), dead blower (61), internal fault (93), flame lost 5 times (13) or unexpected flame (14).

11Rheem

Failed ignition. After four failed attempts the control enters one-hour lockout (10).

13Rheem

Flame lost after established.

14Rheem

Flame present with gas valve off. Extremely critical; furnace will not operate.

30Rheem

Open fuse, usually from a 24 VAC short to common or ground.

33Rheem

Over-temperature (rollout) switch open: flames rolled out of the heat exchanger into the burner compartment.