What does A.O. Smith Tankless E04 Error - Causes & Fix mean?

E04 on an A.O. Smith electric tankless water heater means moisture or a leak is detected inside. Turn off power and water, then call support.

How do I fix A.O. Smith Tankless E04 Error - Causes & Fix?

Follow the step-by-step diagnosis below. The most likely fix is identified by checking power, sensor resistance, and control-board signals in order — using a multimeter and the part numbers documented in the parts reference table.

Can I keep running the equipment with E04 active?

Generally no. Faults on Water heater units are emitted when a sensor, safety, or control circuit is outside its operating envelope. Running through an active fault risks compressor, heater, or control-board damage. Diagnose and clear the fault first.

What part is most commonly replaced for A.O. Smith Tankless E04 Error - Causes & Fix?

The most frequently replaced parts for A.O. Smith Tankless E04 Error - Causes & Fix are listed in the Parts Reference section of this guide with OEM part numbers and typical price ranges. Check the table for direct replacements.

Is A.O. Smith Tankless E04 Error - Causes & Fix a DIY repair or do I need a technician?

Sensor swaps, fan motor replacements, and condenser cleaning are typical DIY-level repairs. Anything involving refrigerant, gas, or high-voltage (>240V) work requires a licensed technician for both safety and code-compliance reasons.

A.O. Smith Tankless E04 Error - Causes & Fix

A.O. Smith Tankless E04 Error — What It Means

The E04 error code on an A.O. Smith electric tankless water heater indicates that the unit has detected moisture or a leak inside the heater cabinet. This is a safety fault that protects the electronics and internal components from water damage. A.O. Smith’s official troubleshooting guidance for E04 is to immediately turn off electrical power and water supply to the unit and contact Technical Support. Do not attempt to reset or continue operation until the source of moisture is identified and corrected.

Jump to Fix

Common Causes

Internal water leak A leak at inlet or outlet connections, or from internal hydraulic components, has allowed water to reach the control or electronics compartment.
Condensation buildup Moisture accumulation inside the cabinet can trigger the moisture detection circuit even without a true plumbing leak.
Water intrusion from outside External water from flooding, plumbing work, or environmental conditions has entered the heater enclosure and wetted sensitive areas.
Failed moisture detection circuit The sensor or wiring that monitors for leaks may have failed or developed a short, causing a false E04 alarm.

Step-by-Step Fix

Shut off electrical power at the branch circuit breakers supplying the tankless water heater.
Shut off the water supply to the heater at the isolation valves on the inlet and outlet lines.
Inspect the unit for visible leaks around all plumbing connections, fittings, and the cabinet interior where moisture could collect.
Dry the interior of the unit thoroughly using towels or a fan, and correct any identified leak source by tightening connections or replacing damaged components.
Do not restore power until the leak source is confirmed repaired and the interior is completely dry.
Contact A.O. Smith Technical Support as directed in the troubleshooting table, since the manual does not provide a field reset procedure for E04.
If the fault persists after the leak is corrected and the unit is dry, Technical Support may authorize control board evaluation or replacement.

Parts Often Needed

Part	Notes
Inlet/outlet plumbing fittings	Amazon \| Replace any cracked or leaking compression fittings or flex connectors identified during inspection.
Control board (if authorized by support)	Amazon \| Contact A.O. Smith for the correct board part number for your model if the moisture fault cannot be cleared.

When to Call a Pro

A.O. Smith explicitly directs that E04 requires contacting Technical Support rather than field repair. If you are not comfortable working with 240-volt electrical circuits or cannot locate the source of the moisture, call a licensed plumber or electrician immediately. Even if you dry the unit and eliminate a visible leak, the fault may not clear without manufacturer-authorized diagnostic steps or component replacement. Because E04 is a safety interlock, do not bypass it or attempt to operate the heater until a qualified technician confirms the unit is safe.