Igniter and spark electrode
The ceramic electrode that throws the spark. When it stops sparking, or sparks across the wrong gap, the unit runs a full ignition cycle and never lights.
What it does
Every call for hot water starts the same way: the fan purges the combustion chamber, the gas valve opens, and the igniter arcs across a precisely set gap to light the burner. Rinnai specifies that gap on its SENSEI units as 0.138 in. (3.5 mm) — not a suggestion, a dimension. Too wide and the spark will not jump; too narrow and it will not reach the gas.
Why it fails
Ceramic cracks. Electrodes erode after tens of thousands of cycles. The mounting gasket hardens and lets the electrode shift, which changes the gap or lets the tip touch the burner and short out. Wiring back to the ignition circuit gets brittle where it passes near hot metal. Rinnai’s own check is to look for a spark at all, then to verify the electrode is tight in its bracket — if you can move it with a finger, the gasket has failed.
Why this is not a DIY part
Same reason as the flame rod: it lives inside a sealed combustion chamber on a gas appliance, its gasket is single-use, and testing it means cycling the unit with the gas on. Rinnai’s own procedure for checking the spark involves holding a live igniter wire with insulated pliers after shutting the gas valve. That is a trained technician’s job.
What a replacement usually costs
The electrode assembly runs $30-120 with its gasket. Since the same visit usually covers the flame rod, the gas pressure check, and the gasket set, budget for a diagnostic call rather than a part swap.