A. O. Smith heat pump water heater
A. O. Smith Voltex hybrid with letter codes (HPTU, 2016–2019) error codes
Covers: Previous generation Voltex / Premier / ProLine XE hybrid with User Interface Module (UIM) and letter codes (2016-2019 manuals; e.g. A. O. Smith HPTU, State HPX-...-DHPT, American HPHE)
Every code listed in A. O. Smith's own manual for this series, with what it means and what A. O. Smith says to do. Last checked 2026-10-03.
Is this your model? For heaters with a User Interface Module that shows letter codes such as EUC, SF, E20 or Edr. The manuals print no model list; A. O. Smith, State and American product pages link this manual for HPTU, HPX-…-DHPT and HPHE models.
| Code | Label | What the manual says it means |
|---|---|---|
| - / -- / --- | System diagnostic | Unit is doing a system diagnostic. |
| ICE | Defrost | Heat pump is in defrosting cycle. |
| HPO | Out of operating range | Ambient temperature <45°F or >120°F, average tank temperature of 59°F or less. |
| EUC | Upper element | Upper element is not functioning. |
| ELC | Lower element | Lower element is not functioning. |
| SF | Air filter | The air filter is dirty. |
| ECF | Compressor short cycling | The heat pump compressor is starting/stopping frequently. |
| E20 or E21 | Upper temperature sensor | Upper Temperature Sensor is not functioning. |
| E30 or E31 | Lower temperature sensor | Lower Temperature Sensor is not functioning. |
| E50 or E51 | Suction temperature sensor | Heat Pump Suction Temperature Sensor is not functioning. |
| E10 or E11 | Coil temperature sensor | Heat Pump Coil Temperature Sensor is not functioning. |
| E40 or E41 | Discharge temperature sensor | Heat Pump Discharge Temperature Sensor is not functioning. |
| E60 | Upper and lower sensors | Upper and Lower Temperature Sensor are not functioning. |
| Edr | Dry fire / tank not full | Not enough water in the tank (tank not full). |
| EPL | Low supply voltage | Power supply voltage is too low. |
| EDH | Discharge temperature high | Compressor Discharge Temperature is too high. |
| EoF | Condensate management | Condensate management. |
| ECL | Suction pressure low | Heat pump suction pressure is too low. |
| EEE | EEPROM failure | EEPROM failure. |
| ECC | Compressor not functioning | Heat pump compressor is not functioning. |
| ECE | Power supply error | Power supply error. |
A. O. Smith's manual for this series does not list error codes; see the troubleshooting section below.
Safety first
- WARNING! Electric Shock Hazard; Disconnect power before servicing. Do not remove the plastic guard from over wiring. Do not touch electrical wiring. Failure to do so can result in death or electrical shock. (diagnostic code pages, p.20-21)
- WARNING! Working near an energized circuit can result in severe injury or death from electrical shock. WARNING! When you are finished, be sure all covers are secured to reduce the risk of fire and electric shock. (p.22)
- If you are not qualified or are not comfortable performing any of the troubleshooting, repair or maintenance procedures, call our Technical Assistance Hotline for further assistance. (p.22)
- WARNING! Do not cap or plug the T&P relief valve or discharge pipe, and do not operate the water heater without a functioning T&P Relief Valve - this could cause an explosion. (p.23)
- WARNING! Be sure the water runs cool before draining the tank to reduce the risk of scalding. (p.26)
- NOTICE! DO NOT turn electrical power back on unless the tank is completely full of water (dry fire precaution). (p.26)
- Diagnostic table note: The diagnostic codes listed are the most common. If a diagnostic code not listed is displayed, contact Residential Technical Support referencing the number on the front of the manual. (p.20-21)
From the manufacturer's manual for this series.
Troubleshooting without a code
No hot water
Likely cause: No power to the water heater (no lights on the unit are on); unit in Vacation Mode; ECO tripped; hot water usage pattern exceeds the capability of the water heater in current mode; non-functioning upper temperature sensor; faulty thermostatic mixing valve; leak in plumbing system.
What to do: Check electrical power to the water heater, check the upper heating element (5-25 ohms) and check/reset the ECO per the manual.
Drips from T&P relief valve discharge pipe
Likely cause: Water pressure too high or need for a properly sized and pressurized Thermal Expansion Tank; large amounts of hot water may be due to tank overheating.
What to do: Install a PRV adjusted to 50-60 psi; install/check a Thermal Expansion Tank; clear debris by manually operating the valve; otherwise have a qualified person replace the T&P relief valve.
Water odor ("rotten egg" smell)
Likely cause: Harmless bacteria in tap water multiplying in the water heater.
What to do: A higher tank temperature setting of 140° kills the bacteria; install a properly adjusted Thermostatic Mixing Valve at each point-of-use. In some cases the standard anode rod can be replaced with a special zinc anode rod which may help; contact a qualified person. An anode rod must be installed at all times or the warranty is void.
Water heater sounds
Likely cause: Normal expansion/contraction; sediment buildup on or around the elements; heat pump compressor or fan running.
What to do: Drain and flush the tank as directed under "Draining and Flushing".
Temperature too high
Likely cause: Set point too high; nonfunctioning thermostat or shorted heating element.
What to do: Adjust the UIM setting; install or adjust a Thermostatic Mixing Valve. Turn power off until this problem is fixed if very hot water is released.
Low water pressure (hot side)
Likely cause: Melted dip tube from soldering; partially closed supply valve.
What to do: Replace the dip tube if melted; open the water heater's supply valve fully.
Owner maintenance
maintenance schedule summary
How often: after first six months of operation and at least annually thereafter
Drain and flush tank, clean air filter, clean condensate pan, and remove and inspect anode rod after first six months of operation and at least annually thereafter. Operate the T&P Relief Valve annually and inspect T&P valve every 2-4 years (see the label on the T&P valve).
drain and flush tank
How often: after the first six months, then at least annually depending on water hardness, or more frequently if needed
Turn breaker OFF; run hot water until cool; connect a hose to the drain valve; turn cold supply OFF; open drain valve and a hot faucet; flush until no more sediment; refill completely before restoring power. DO NOT turn electrical power back on unless the tank is completely full of water.
inspect/replace anode rod
How often: after the first six months, then annually or more frequently if needed (more often with a water softener)
Replace if substantially worn out or depleted. Obtain new anode rods from your local plumbing supplier or have a qualified person replace it (consumable item, not covered under warranty).
clean or replace air filter
How often: when the UIM displays "SF"
Before cleaning, turn off power at the circuit breaker/fuse box. 1. Take the two tabs on the air filter and remove (slide) it from the top cover of the unit. 2. Clean with a vacuum with a hose attachment (or replace). 3. Place the new or cleaned filter into the water heater. 4. Restore power.
clean condensate drain pan and lines
How often: not stated in this section (summary on p.3: at least annually)
Before cleaning, shut off power at the circuit breaker/fuse box. 1. Remove the top shroud by loosening the screws. 2. Check the condensate drain pan and drain lines for dirt or debris; wipe out with a damp cloth. 3. Secure the shroud back. 4. Restore power.
Sources
- Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide (100323093_2000578009)
- Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide (State, hotline 1-800-821-2017) (100270591)
- Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide (American, hotline 1-800-999-9515) (100270579)
- Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide (Lowe's / AOSmithAtLowes edition, hotline 1-877-817-6750) (2000536048 (VER 00) 100277324 (REV A))