Takagi / A.O. Smith tankless · Water Heater error code

651Flow adjustment valve fault

Cross-checked Power off, standard tools

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Related diagnostic guides

Guide · Water Heater

Field pattern: tankless water heater goes cold mid-shower with no code (flow, inlet filter and scale first)

A tankless heater only fires above a minimum flow rate and drops out when flow falls near that line. Clogged inlet screens, scaled heat exchangers, low-flow showerheads and aerators, and mixing valves that cut hot flow are the usual causes. The wrong turn is calling it a gas or ignition fault and replacing parts when the real issue is water flow.

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Tankless: error code, no ignition at low flow, or temperature swings

Many tankless complaints are water-side: flow below the activation threshold, a clogged inlet screen, low-flow aerators, or scale in the heat exchanger. Gas supply, venting and combustion codes are blocked: diagnose and refer. Read the code first, then work the water side.

6 steps ~90 minmoderate