A. O. Smith heat pump water heater
A. O. Smith Early Voltex hybrid (FPTU, 2015 manual) error codes
Covers: Earlier Voltex hybrid with UIM (April 2015 manual; linked from A. O. Smith FPTU product pages)
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 the earlier Voltex FPTU-50/66/80, whose product pages link the April 2015 manual. It uses the same letter codes as the 2016–2019 heaters, with some differences in what the manual says to do.
| 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-120°F, average tank temperature of 59°F or less. [sic, as printed] |
| 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. |
| Edr | Dry fire | Not enough water in the tank (tank not full). This is also referred to as "Dry Fire". |
| EPL | Low supply voltage | Power supply voltage is too low. |
| EDH | Discharge temperature high | Heat Pump Discharge Temperature is too high. |
| EoF | Condensate pump failure | Condensate pump failure (if Accessory Condensate Pump is installed). |
| 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! Working near an energized circuit can result in severe injury or death from electrical shock. (troubleshooting/maintenance sections)
- NOTE: The diagnostic codes listed are the most common. If a diagnostic code not listed is displayed, contact Residential Technical Assistance referencing the number on the front of this manual. (p.20-21)
From the manufacturer's manual for this series.
Troubleshooting without a code
No hot water
Likely cause: No power (no lights on); unit in Vacation mode; ECO open; usage pattern exceeds capability in current mode; non-functioning upper temperature sensor; faulty thermostatic mixing valve; leak in plumbing system.
What to do: Check for blown fuse or tripped breaker; press Mode/Enter and return to desired operating mode; reset the high temperature limit switch (see "Safety shut-off"); change mode or modify usage; contact a qualified person for service (sensor); check hot water at other faucets; check hot water side of plumbing for leaks.
Insufficient hot water / slow hot water recovery
Likely cause: Set point too low; air filter dirty; usage exceeds capability in current mode; water connections reversed; heat lost through long run of exposed pipe; hot water leak; non-functioning heating element; sediment or scale build up; incorrectly installed outlet j-tube; thermostatic mixing valve faulty/set too low; low supply voltage.
What to do: Increase set point; clean air filter; change mode (e.g. switch to Hybrid Mode); ensure cold connection at bottom and hot at top; insulate exposed piping; repair leaks; call qualified person for service; drain and flush tank (water conditioning may be necessary); check j-tube alignment mark; check other faucets; check power (voltage).
Temperature too high
Likely cause: Non functioning thermostat; grounded/shorted heating element; thermostatic mixing valve faulty.
What to do: Replace thermostat; replace heating element; check hot water at other faucets.
Low water pressure
Likely cause: Partially closed supply valve.
What to do: Open supply valve completely.
Water odor
Likely cause: Sulfate in supply water; little or no dissolved oxygen; sulfate reducing bacteria in the heater (nontoxic); excess active hydrogen from the anode's corrosion-protective action.
What to do: Replace anode.
Sounds
Likely cause: Normal expansion/contraction; sediment buildup on or around the elements; heat pump compressor or fan running.
What to do: No action required; drain and flush the tank as directed under "Draining and Flushing"; no action required.
Drip from Temperature & Pressure Relief Valve
Likely cause: Excessive water pressure; thermal expansion; non-functioning T&P relief valve; debris under valve seat.
What to do: Check water supply inlet pressure; if higher than 80 PSIG install a pressure reducing valve (50-60 PSIG recommended); add or service a thermal expansion tank; replace the T&P relief valve; see "Drips from T&P Relief Valve Discharge Pipe".
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.