A.O. Smith Water Heater E4 Error Code — What It Means
On A.O. Smith electric tankless water heaters, the E4 or E04 code indicates that the control system has detected moisture or a leak inside the heater cabinet. This is a safety lockout that prevents further operation until the source of water intrusion is corrected. The manufacturer requires you to turn off both electrical power and the water supply immediately when this fault appears. Other A.O. Smith product lines or models may use E4 for completely different faults, so always confirm your exact model number and manual before starting any repair.
Common Causes
- Active water leak inside the cabinet A fitting, valve, heat exchanger seal, or plumbing connection has failed and is dripping onto control components or the leak sensor.
- Residual moisture after a previous leak or service Water pooled in the cabinet or on wiring after the original leak was repaired, and the sensor still detects dampness even though the active leak is gone.
- Condensation buildup High humidity or poor ventilation causes condensation inside the heater enclosure, triggering the moisture sensor.
- Corroded or shorted sensor wiring Past water exposure has damaged connectors or created a short circuit that the control board interprets as a moisture event.
- Failed moisture sensor or control board The sensor itself or the control board circuitry that reads it has failed and reports a false moisture condition.
Step-by-Step Fix
- Turn off power at the circuit breaker and shut the cold-water supply valve to the heater before you open any panels or touch any components.
- Remove the front cover and inspect the entire cabinet for visible water, wet insulation, damp wiring, or pooled moisture around fittings and the heat exchanger.
- Identify and repair any active leak by tightening compression fittings, replacing leaking valves, or resealing heat-exchanger connections before you proceed.
- Dry the interior thoroughly using a shop vacuum and clean towels, then leave the cover off in a well-ventilated space for several hours to make sure all moisture evaporates.
- Inspect all wire connectors and sensor contacts for corrosion or water stains, and clean or replace any damaged terminals or harnesses.
- Restore power and water supply and monitor the display for five minutes to see whether the E4 code clears or returns.
- Contact A.O. Smith technical support if the fault persists after confirmed drying and no visible leak, because the manual directs you to professional diagnosis for persistent E04 codes.
Parts Often Needed
| Part | Notes |
|---|---|
| A.O. Smith moisture sensor or leak detector assembly | Amazon | Only if technical support confirms sensor failure after all moisture is removed and the fault remains. |
| A.O. Smith electric tankless control board | Amazon | Required when the board circuitry is shorted or corroded by past water intrusion and replacing the sensor does not clear the code. |
| Wire harness or connector kit | Amazon | For corroded or damaged sensor and control wiring that cannot be cleaned. |
When to Call a Pro
Call a licensed plumber or A.O. Smith service technician if you cannot locate the source of the leak, if the E4 code returns immediately after drying and you have confirmed zero visible moisture, or if you see widespread corrosion on the control board or wiring harness. The manufacturer explicitly instructs you to contact technical support when E04 appears, so professional diagnosis is the approved path if basic drying and leak repair do not resolve the fault. Also call a pro if your water heater is still under warranty, because unauthorized component replacement may void coverage.