Flame rod (flame sensor)
The thin metal rod that proves to the control board that a flame is actually burning. Cheap to buy, awkward to reach, and behind more ignition-related error codes than any other single component.
What it does
A tankless heater does not trust its own igniter. After it opens the gas valve it needs proof that gas is actually burning, and the flame rod is that proof: a small current passes from the rod through the flame to ground, and the control board reads it. Rinnai’s service documentation puts the minimum at 1 microamp — below that, the board assumes there is no flame and shuts the gas off within seconds.
That is the behaviour you experience as a unit that fires, runs for a moment, and quits.
Why it fails
Almost never electrically. What kills a flame rod is what settles on it: carbon from incomplete combustion, or the white deposit that comes from burning gas in air carrying dust or certain chemicals. Either coats the rod and insulates it from the flame it is meant to sense. A rod that has worked loose in its bracket does the same thing — Rinnai calls this a floating flame rod and lists it as a direct cause of ignition faults.
Why this is not a DIY part
The rod itself costs less than a restaurant meal, and on many appliances cleaning a flame sensor is a standard homeowner job. Not here. On a condensing tankless unit the rod sits inside a sealed combustion chamber, and its gasket must be replaced — never reused — any time the assembly is disturbed. Getting that seal wrong on a gas appliance is how combustion products end up somewhere they should not be. Pay for the visit.
What a replacement usually costs
The part is $15-70 with its gasket. The bill is the diagnosis: measuring micro-amp output, checking the electrode gap, and ruling out the gas valve and control board, which is why flame-sensor work on a tankless unit commonly lands in the $200-500 range all-in.