12 Flame failure

What the control is telling you

The flame went out or was lost after ignition. If code 12 shows right as water starts flowing with no visible flame (SENSEI), suspect an electrical short in a DC circuit component.

Applies to: SENSEI, SENSEI RX/CX and non-SENSEI tankless

Likely causes

• Gas off, propane tank empty, or inlet pressure low from undersized piping
• Loose electrode or flame rod, wrong electrode gap, or damaged flame rod wiring
• Cracked or heat-damaged flame rod silicone sleeve letting the spark ground outside
• Poor grounding or wrong polarity in the electrical circuit
• Vent problems or partly engaged vent joints causing flue gas recirculation
• Debris or insects in the fan wheel or intake
• Moisture in wiring harnesses, or a short in a DC component (harness, flow control, bypass valve)
• Faulty gas valve, regulator, MSA/MSB board, or wrong orifice or gas parameter

Test and repair steps

1. Warning: Only gas-qualified technicians should do these checks.
2. Make sure gas is on at the heater, meter or propane tank, and the propane tank has gas. Measure inlet pressure with all gas appliances at high fire. The rating plate minimum must be held.
3. If pressure is low, check gas line sizing (max drop 0.3 in. W.C. NG / 0.5 in. LP on black iron, up to 14 in. W.C., per NFPA 54). Have a gas sizing professional check if exceeded.
4. Inspect the vent for loose joints, wrong fittings and terminal clearances. Confirm equivalent length and approved materials. On concentric/common vent, mark and separate joints to confirm full insertion.
5. Make sure the electrode and flame rod cannot move by hand. If loose, replace the electrode/flame rod assembly and gasket. Set the electrode gap to 0.138 in. (3.5 mm).
6. Check flame rod wiring and connectors at the rod and PC Board. Replace the silicone sleeve if it is cracked or heat-damaged, because the spark can ground outside the chamber.
7. Check that all ground connections are tight, clean and not corroded, and polarity is correct. Use a licensed electrician if unsure.
8. Warning: Turn off power before unplugging or reconnecting the fan motor, or it may short. Check the blower wheel for debris or insects. If found, inspect the whole intake path including the vent.
9. Two-stage regulator must be at least 6 ft away. With the heater off, static inlet pressure must not exceed 10 in. W.C. (NG) or 13.5 in. (LP), or the regulator may be faulty.
10. Check harnesses for damp or wet connections, which can short and cause code 12. Dry them and find the source.
11. Isolate the unit from any MSA/MSB system. If it runs without code 12, suspect the MSA/MSB board.
12. The gas valve may be blocked or faulty, so replace it if needed. SENSEI/RX/CX: measure the solenoid (SENSEI conn "D" pins 27/29 = 18–22 Ω or 11–13 VDC; RX/CX "CN9" pins 3/4 = 11–13 VDC).
13. SENSEI confirm orifice color (red = LP, blue = NG). SENSEI RX/CX: confirm the gas parameter (10A = NG, 10b = LP).
14. SENSEI/RX/CX, immediate code 12 or 19 with no flame: there is a DC short. Unplug one component at a time and cycle the unit. If it cycles 3 times, the part you just unplugged is shorted.

Settings for these units

• RSC supply connection sizes · SENSEI
• RSC water quality limits · SENSEI
• RSC clearances to combustibles · SENSEI
• RSC temperature settings available · SENSEI
• RX supply connection sizes · SENSEI RX/CX
• RX water quality limits · SENSEI RX/CX
• RX water treatment and flushing by hardness · SENSEI RX/CX
• RX clearances to combustibles · SENSEI RX/CX