120 Flame failure (flame lost during operation)

What the control is telling you

The flame goes out while running; the unit tries to relight, and if flame is lost again after a successful relight it locks out with code 120. Immediate 120 with no flame points to a DC short.

Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi and Solo)

Likely causes

• Wrong gas type (A0) or altitude (A3) parameter, or wrong orifice (175K+)
• Gas off, undersized propane supply, air in lines, or low/unstable inlet pressure
• Venting problems, blocked terminations or flue gas recirculation
• Dirty, loose or mispositioned spark electrode/flame rod
• Damaged ignition/flame rod wiring, sleeve or poor grounding
• Moisture in wiring connections
• Electrical short in a DC component (immediate code with no flame)

Test and repair steps

1. Warning only a qualified gas technician should perform these checks.
2. Parameters confirm A0 gas type (A = Natural Gas, b = Propane) and A3 altitude (A = 0–2,000 ft, B = 2,001–5,400 ft, C = 5,401–7,700 ft, D = 7,701–10,200 ft).
3. 175K and higher confirm the correct gas orifice is fitted in the gas valve, groove side out (grey = propane, black = natural gas, white = natural gas high altitude above 2000 ft).
4. Gas confirm gas is on at unit, meter and/or tank; propane tank and piping sized correctly; lines purged of air; any two-stage regulator placed per its maker.
5. Connect a manometer to the bottom test port: confirm stable static pressure, then dynamic inlet pressure within rating-plate spec with all gas appliances on high fire.
6. Venting check materials, length, room air if non-direct vent (approx. 50 cu ft per 1,000 BTU/h), clear terminations and clearances; on Rinnai concentric/common vent, confirm joints fully inserted (recirculation = 120).
7. Warning (175K and higher) don't pull electrodes straight out — they're bent 90°; withdraw slowly while rotating toward the burner door. Always fit a new gasket when removing spark electrode or flame rod.
8. Watch through the burner window for spark and flame; a lit flame should contact the flame rod. If flame drops out, suspect the flame rectification circuit or gas supply.
9. With the gas valve off confirm PC Board ground to cabinet and all grounds tight, clean and correctly polarized; confirm electrode position and gap of 0.138 in. (3.5–4 mm) with no arcing to the burner surface.
10. Ensure electrode and flame rod are tight; if loose, gently clean carbon/white deposits without scratching. Check wiring to ignitor module and PC Board; replace a cracked or heat-damaged silicone sleeve.
11. Check PC Board outputs ignitor feed at CN8 pin 2 (black) to pin 3 (white) = 11–14 VDC during ignition; gas solenoid at CN8 pin 11 (black) to pin 12 (yellow) = 15–25 Ω or 11–14 VDC.
12. With power, gas and water off, check all harness connections for moisture; dry them and find the source.
13. Check for heat exchanger leaks (liquid at the condensate drain while idle). Power off before handling the fan harness, then inspect fan wheel and intake path for debris/insects.
14. If the gas valve may be sticking due to condensate, debris or internal fault, replace it. If 120/190 shows immediately with no flame, unplug DC components one at a time to locate a short.