Flow sensor (water flow turbine)
The small turbine that tells the heater water is moving. Nothing happens until it does — a tankless unit will not fire at all if this sensor says there is no flow, which is why a failed one looks exactly like a dead water heater.
What it does
A tankless heater has no tank to keep warm, so it has to know the instant you open a tap. The flow sensor is what tells it: a small turbine in the cold water inlet whose spin rate the control board reads as gallons per minute.
That reading does two jobs. It starts the ignition sequence, and it sets how hard the unit fires — the board compares flow against inlet and outlet temperature to decide how much gas to burn. Rinnai’s own no-code troubleshooting starts by reading the flow rate off the controller’s performance data, because a wrong number there explains most of the symptoms people bring to a heater.
It is also why every manufacturer publishes a minimum activation flow rate. Below it, the turbine is not turning fast enough for the unit to commit, and the heater sits there doing nothing while the tap runs cold.
Why it fails
Debris and scale, mostly. The turbine is a moving part in the water path, so anything that gets past the inlet filter ends up here — grit from a water main repair, scale flaking off a heat exchanger, pipe dope from an installation. Rinnai’s check is simply whether the turbine still spins freely.
The other failure is electrical: the sensor reports open or short circuit rather than a wrong number. Navien gives that its own codes, and on the NPE-2 it narrows one of them to installations with an air handler.
The cheap thing to try first
Clean the inlet water filter. It will not repair a failed sensor, but a clogged filter produces the same symptom — a heater that will not fire because it cannot see enough flow — and it is a ten-minute job with no parts. On a fair number of “the flow sensor has failed” call-outs, the filter was the whole story.
How to clean the inlet water filter
Why this is not a DIY part
The sensor sits in the water path inside the cabinet, so the unit has to be isolated and drained to reach it. On several models the diagnosis is a voltage reading at specific pins on the control board, which is what separates a failed sensor from a failed board — and replacing the wrong one of those two is an expensive mistake.
What a replacement usually costs
$40-160 for the sensor. Expect $150-500 installed, most of which is the visit and the diagnosis rather than the component.