A. O. Smith heat pump water heater
A. O. Smith Voltex MAX / Signature 900 Premium (HPTA, HPA10) error codes
Covers: Current generation, Premium / MAX model with Smart Valve (A. O. Smith Voltex MAX HPTA / Signature 900 HPA10 at Lowe's; State Premier MAX HPAX; American ProLine MAX HPA102xx)
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? Covers A. O. Smith HPTA-40/50/66/80 and Signature 900 HPA10 (Lowe's), State HPAX, and American HPA102xx models, which have the Smart Valve. The display shows 3-digit codes.
| Code | Label | What the manual says it means |
|---|---|---|
| 001 | Dry-fire | Dry-fire, electrical power on with the tank not completely full of water. |
| 003 | Upper thermistor sensor failure | Upper thermistor sensor failure. Upper thermistor sensor is mounted to tank and located above upper element. NOTICE: The Control Assembly will go into Limp Mode 1 until the failure is corrected. |
| 004 | Lower thermistor sensor failure | Lower thermistor sensor failure. Lower thermistor sensor is mounted to tank and located above lower element. NOTICE: The Control Assembly will go into Limp Mode 2 until the failure is corrected. |
| 006 | Internal Processor Error | Internal Processor Error - Frequency Fault - Standard AD Reference Fault - Non-Volatile Memory Fault - Crystal Fault. |
| 009 | Power supply voltage too low | Power supply voltage is too low. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 021 | Upper element circuit failure | Upper element circuit failure. Lower element is still operable. NOTICE: The Control Assembly will go into Limp Mode 1 until the failure is corrected. |
| 022 | Lower element circuit failure | Lower element circuit failure. Upper element is still operable. NOTICE: The Control Assembly will go into Limp Mode 2 until the failure is corrected. |
| 025 | Heat pump coil temperature sensor | Heat Pump Coil Temperature Sensor is not functioning. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 026 | Heat pump suction temperature sensor | Heat Pump Suction Temperature Sensor is not functioning. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 027 | Heat pump discharge temperature sensor | Heat Pump Discharge Temperature Sensor is not functioning. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 028 | Ambient temperature sensor failure | Ambient Temperature Sensor failure (Heat Pump Compressor is outside of operational temperature range). NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 029 | Outlet temperature sensor failure | Outlet temperature sensor failure. NOTICE: Guest Mode and Hot Water+ Mode will be unavailable until the failure is corrected. |
| 031 | Water leak | Water Leak. |
| 044 | SAC anode depleted | SAC anode depleted. |
| 046 | Shut-off valve | Shut-off Valve (if applicable). |
| 047 | Smart Valve failure | Smart Valve failure. NOTICE: Guest Mode and Hot Water+ Mode will be unavailable until the failure is corrected. |
| 048 | Battery low energy | Battery Low Energy. |
| 080 | Air filter dirty | Air filter is dirty. |
| 081 | Condensate management | Condensate management. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 083 | Compressor suction pressure too low | Heat Pump Compressor suction pressure is too low. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 084 | Compressor not functioning | Heat Pump Compressor is not functioning. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 085 | Compressor discharge temperature too high | Heat Pump Compressor discharge temperature is too high. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| 086 | Fan speed feedback | Fan speed feedback. NOTICE: The Control Assembly will go into Limp Mode 3 until the failure is corrected. |
| HPO | Heat pump out of operating range | The ambient and/or water temperature is outside the heat pump's operating range. The heat pump will not operate at ambient temperatures below 37°F or above 120°F, or if the average water temperature in the tank is below 55°F. The unit operates in Electric Mode until temperatures return to the heat pump's range. |
| ICE | Heat pump defrost | Frost builds up on the evaporator when the heat pump runs in low ambient temperatures. The controller puts the unit into a defrosting cycle, and the display shows ICE during the defrosting period. |
| CLR | Error code cleared | Shown when an error code is temporarily cleared from the display. |
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. (p.28)
- WARNING! When you are finished, be sure all covers are secured to reduce the risk of fire and electric shock. (p.28)
- 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, or contact a qualified person. (p.28)
- *These instructions are brief and intended as guidance for a qualified person. If you lack the necessary skills to perform these procedures call the Technical Assistance Hotline which is listed on the water heater's warranty sheet for assistance. (code chart footnote, p.27)
- A tripped ECO can usually be reset, but you should have a qualified person investigate the cause of the overheating. Do not turn the power back on until the cause of the overheating has been identified and repaired. (p.29)
- 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.29)
- WARNING! When manually operating the T&P Relief Valve, make sure that no one is in front of or around the discharge outlet. The water may be extremely hot and could cause severe burns. (p.29-30)
- WARNING! Be sure the water runs cool before draining the tank to reduce the risk of scalding. (p.32)
- NOTICE: DO NOT turn electrical power back on unless the tank is completely full of water (protects elements against dry fire). (p.32)
- CAUTION! Battery may explode if mistreated. DO NOT recharge, disassemble or dispose of in fire. (p.37)
- Do not attempt to operate this water heater if the unit has been submerged, subjected to flooding, or surrounding insulation has been exposed to water in any way; flood-damaged units must be replaced. (Service Handbook 2000620230, p.12)
From the manufacturer's manual for this series.
Troubleshooting without a code
Not Enough Hot Water (no error code displayed)
Likely cause: High usage, plumbing leak, operating mode adjustment
What to do: 1. Check for plumbing leak. 2. Adjust temperature; see scald warnings on heater and in manual. 3. Contact a qualified person to perform a volume test.
Water Too Hot (no error code displayed)
Likely cause: Water temperature set too high or grounded element
What to do: 1. Reduce temperature setting; or 2. Turn OFF electrical power at breaker. 3. Check element circuits for resistance of 5-25 ohms (replace if required). 4. Reconnect/turn on power. 5. If the error persists, contact a qualified person.
No Hot Water (no error code displayed)
Likely cause: No power, control panel or thermostat operation
What to do: 1. Turn OFF electrical power at breaker. 2. Unplug and reconnect 24 pin connector. 3. Reconnect/turn on power. 4. Contact a qualified person to verify correct voltage to unit. NOTICE: If the control panel was not connected via Wi-Fi or Bluetooth, the time will need to be set.
Limp Mode (heater keeps running on one heating method)
Likely cause: Limp Mode 1: upper element or upper thermistor failed (lower element satisfies upper call for heat). Limp Mode 2: lower element or lower thermistor failed (upper element satisfies). Limp Mode 3: heat pump unavailable (both elements satisfy the call for heat).
What to do: The Control Assembly will operate in Limp Mode until the failure is corrected.
No hot water
Likely cause: No power to the water heater (no lights on the unit are activated); 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 the electrical power to the water heater, check the upper heating element (good element reads 5-25 ohms), check/reset the Energy Cut Off (ECO). A tripped ECO can usually be reset, but you should have a qualified person investigate the cause of the overheating and repair the problem. Do not turn the power back on until the cause of the overheating has been identified and repaired.
Drips from T&P relief valve discharge pipe
Likely cause: A small amount of dripping usually means the home's water pressure is too high or you need a properly sized and pressurized Thermal Expansion Tank; a large amount of hot water may be due to the tank overheating; in rare cases debris under the valve seat.
What to do: Install a Pressure Reducing Valve (PRV) on the main cold water supply line and adjust to 50-60 psi; install (or check the pressure/bladder of) a Thermal Expansion Tank; you may be able to clear debris by manually operating the valve (see the label on the T&P Relief Valve). If pressure is 50-60 psi, an expansion tank is installed and pressurized, debris is cleared and it still drips, have a qualified person replace the T&P Relief Valve.
Water odor ("rotten egg" smell)
Likely cause: Harmless bacteria normally present in tap water can multiply in water heaters.
What to do: A higher tank temperature setting of 140°F/60°C kills the bacteria that causes "smelly water"; a properly adjusted Thermostatic Mixing Valve should be installed at each point of use. NOTICE: an anode rod must be installed at all times or the warranty is void.
Water heater sounds
Likely cause: Normal expansion and contraction of metal parts; sediment buildup on or around the elements; the heat pump compressor or fan running.
What to do: Sediment buildup can create noise and may cause premature tank failure: drain and flush the tank as directed.
Temperature too high
Likely cause: Set point too high; a non-functioning thermostat or a shorted heating element can cause extremely hot water.
What to do: Adjust the Control Assembly; install or adjust a Thermostatic Mixing Valve. If the T&P Relief Valve releases large amounts of very hot water, turn power OFF until this problem is repaired.
Low water pressure (hot side only)
Likely cause: Melted PEX liner (from soldering pipes connected to the heater) or partially closed supply valve.
What to do: Replace the nipples at the hot and cold water connections if the liner is melted; open the water heater's supply valve fully. If both hot and cold have low pressure, call your local water utility.
Insufficient or no hot water, or slow recovery
Likely cause: Thermostatic mixing valve faulty or set too low; set point too low; usage exceeds capability in current mode; reversed water connections; plumbing leak; lower element not working; no power; Vacation Mode; ECO tripped; non-functioning upper thermistor (code 003); dirty air filter; out of position outlet j-tube.
What to do: Check each cause; e.g. adjust set point, clean the air filter, repair leaks, re-install reversed connections, replace the lower element if necessary.
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. Inspect and operate the T&P Relief Valve annually (see the label on the T&P valve for maintenance schedule).
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 power OFF; open a hot water faucet and let hot water run until cool; connect a garden hose to the drain valve; turn the cold water supply valve OFF; open the drain valve (opening a hot faucet helps); if sediment was present, flush by opening the cold supply until no more sediment drains; close drain valve; refill with a hot faucet open, running full at least three minutes so the tank is completely full; then restore power. DO NOT turn electrical power back on unless the tank is completely full of water.
inspect/replace anode rod
How often: inspect after the first six months, then annually or more frequently if needed (more often with a water softener); Control Assembly displays code 044 when depleted
Turn power OFF, run hot water until cool, turn cold supply OFF, open a hot faucet to depressurize; remove ducting and the jacket cover; remove the top nut (10 mm) and wire terminal, then remove the anode with a 1-1/16" socket and extension; replace if depleted; reinstall with thread sealant, refill completely, check for leaks, restore power. Or have a qualified person replace it (anode rods are consumable and not covered under warranty).
clean or replace air filter
How often: when the Control Assembly displays code 080
Before cleaning, turn OFF power to the water heater at the circuit breaker/fuse box. 1. Take the tab on the air filter and remove (slide) it from the top facing air intake duct adaptor. 2. Clean with a vacuum with a hose attachment. 3. Slide the new or cleaned filter into the adaptor. 4. Restore power.
clean condensate drain
How often: not stated in this section (summary on p.3: at least annually)
Before cleaning, turn OFF power to the water heater at the circuit breaker/fuse box. 1. Check the condensate drain pan and drain lines for dirt or debris. 2. Use a pipe cleaner to clear debris from condensate drain, condensate line, and condensate drain pan. 3. Restore power.
T&P relief valve
How often: at least annually
Inspect the T&P Relief Valve and discharge pipe and operate the valve manually to ensure waterways are clear (follow the yellow label on the valve). If water does not flow freely, turn the power OFF and call a qualified person.
inspect heating elements
How often: at least annually
Inspect upper and lower elements for scaling or damage; if elements show scaling or damage they MUST be replaced.
replace Control Assembly battery
How often: every five (5) years recommended
Slide out the battery tray on the left side of the Control Assembly (Phillips screw) and replace the battery (see code 048 for battery type).
Sources
- Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide (Premium/Smart Valve) (100379654_2000812401)
- Installation Instructions and Use & Care Guide (Lowe's edition, Signature 900 HPA10) (100379656_2000812402)
- Residential Hybrid Electric Heat Pump Water Heater Service Handbook (for qualified service professionals only) (2000620230 REV.D)
- Replacing the Anode Rod (instruction sheet) (100392298)