AO Smith 701: Control Board Fault

Unlike most AO Smith tankless codes, 701 isn't pointing at gas, flame, or venting — it's the main control board itself reporting that something about its own operation doesn't check out.

Applies to: AO Smith tankless/condensing water heaters

What it means

Computer (Control) Board Fault — the main control board has detected an internal problem. The board runs every other system in the unit, so a fault here can stem from something as simple as a loose wire feeding it bad information, or as final as a failed component on the board itself — which is exactly why the one homeowner step worth trying is a full power cycle before assuming the worst.

Most common causes

Broken or loose wiring to the control board
A sensor reading that's confusing the board's logic
Board component failure

What you can safely check yourself

Power-cycle the unit and see if the fault clears

When to call a professional

The fault returns after a power cycle — the control board likely needs professional replacement.

Control board diagnosis and replacement involves line-voltage wiring and internal gas ignition circuitry, and should be handled by a licensed HVAC technician.

What the fix costs

A main control board replacement is one of the pricier tankless repairs, typically about $250–$600 installed according to HomeGuide's and Angi's breakdowns of water heater component costs — parts availability for your specific model can push that toward the higher end. Before authorizing a board swap, confirm the technician has actually tested the board rather than replacing it as a guess, since loose wiring or a bad sensor can mimic a board fault at a fraction of the cost.

Related codes

All AO Smith codes

Every AO Smith water heater fault code — the full list on one page. Not sure the code on your display is really 701? Water heater error code flashing or blinking walks through reading a tankless code correctly before you act on it.

Safety. A single power cycle (turning the unit off, waiting, and back on) is safe to try yourself. Beyond that, do not open the cabinet or probe the control board's wiring — it carries line-voltage power and connects directly to the gas ignition system, and should only be handled by a licensed technician with power off.

Sources

AO Smith tankless water heater error code guide (manufacturer documentation)

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.