E19
A.O. Smith Accessory / Sensor Bus Communication Fault

A.O. Smith heater shows E19; unit may not recognize sensors or accessories

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Loose accessory bus connector, Incorrectly wired external sensors (e.g., remote temperature sensor), Faulty accessory module, Control board bus failure

How to Fix / Troubleshooting

Safety: Disconnect power before working on low-voltage wiring.

  • 1. Identify accessories: Determine which external devices are connected (remote thermostat, building management interface, external tank sensor, etc.).
  • 2. Check connections: Inspect the low-voltage terminal strip and accessory connectors. Tighten screws and re-seat plugs.
  • 3. Remove add-ons: Temporarily disconnect non-essential accessories one at a time to see if E19 clears. A faulty module can bring down the bus.
  • 4. Verify wiring polarity: Ensure any two-wire communication or sensor lines are connected with correct polarity as specified by A.O. Smith.
  • 5. Replace faulty module: If a specific accessory causes E19 when connected, replace that module. If no accessory is at fault, the main control board may need replacement.
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Repair Difficulty

Medium 3/5

Required Part

Accessory Interface Harness
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