How to Fix: IMPORTANT: Leave the area the water heater is installed in until safety personnel have identified the area as safe and all flammable vapors have been eliminated.
Inspect flammable vapor sensor to confirm it’s not damaged and check wiring harness for loose, damaged, or broken connections. Once 100% sure no flammable vapors are present, turn power and gas back on, and open a hot water tap to fire the unit back up. If code reappears, replace the flammable vapor sensor and/or PC board.