AO Smith Water Heater Error Code 084: Heat Pump Compressor Not Working (Meaning, Causes, Fixes)
What the code means
In A. O. Smith's Residential Hybrid Electric Heat Pump Water Heater Service Handbook, diagnostic code 084 with a flashing alert icon and red LED is listed as Heat Pump Compressor is not functioning. It sits in a group with 083 (compressor suction pressure too low) and 085 (compressor discharge temperature too high), and all three share the same Compressor Check flowchart. The beeper only sounds if the code is triggered three times in one hour, so a quiet 084 may have happened once or twice. The handbook covers the A. O. Smith HPTS-50/66/80, HPTA-40/50/66/80 and 120V HPTV-50/66/80 models, the HPS10, HPA10 and HPV10 retail versions, and the matching American, Lochinvar, Reliance and State models. Don't confuse 084 with HPO: the handbook says HPO is a normal message when the air is below 37°F or above 120°F or the tank water is below 59°F, and the unit then heats with its electric elements until conditions return to the heat pump's range. Older A. O. Smith heat pump models use letter codes instead, where the same fault is shown as ECC.
Likely causes
- The compressor overheated and its internal thermal switch opened. The handbook's first step is to check whether the compressor shell is hot and let it cool so the switch resets
- The fan isn't running, so the compressor can't move heat. The flowchart sends you to the separate Fan Check if the fan doesn't start
- A dirty air filter or evaporator coil. The flowchart's fix for a compressor that runs for more than 5 minutes and then faults includes cleaning the filter and coil
- Loose or damaged wiring at the compressor terminals or at connector CN3 on the daughter board
- A failed run capacitor (the flowchart has the technician discharge it and test its capacitance)
- A thermistor reading out of range (ambient, discharge, coil, suction, or the upper and lower tank thermistors)
- No supply voltage reaching the compressor from the daughter board, or a failed compressor. The handbook says to contact Tech Support if every check passes, and that the water heater should then be replaced
Safe DIY checks
Turn off power at the furnace/HVAC service switch and the breaker before opening panels.
- Note whether it says 084 or HPO. HPO in a cold garage or basement is normal and needs no repair; 084 is a compressor fault.
- Turn off power to the water heater at the breaker. If you can safely reach the heat pump section, the handbook's first check is whether the compressor shell is hot; if so, leave the power off and let it cool so its thermal switch can reset.
- Clean the air filter on top of the unit: slide it out of the air intake duct adaptor by its tab, vacuum off the dust and debris with a hose attachment, and slide it back in (A. O. Smith's steps for the dirty-filter code 080).
- Turn the power back on. The handbook says the water heater runs a system diagnostic of about 7 to 10 minutes each time it's powered up (alternating dashes on the display), and then the fan starts. Make sure the fan actually runs.
- If you need hot water while you wait for service, the handbook describes Electric Mode, which runs the unit as a conventional electric water heater using only the elements, so the compressor isn't needed. Ask A. O. Smith support whether that's right for your unit.
- If 084 comes back, call the Technical Assistance Hotline on your water heater's warranty sheet. A. O. Smith says its service instructions are meant for a qualified person, and the compressor area has high voltage and pressurized refrigerant terminals.
🛠 When to call a pro
- 084 returns after a power cycle, a cool-down and a clean filter.
- The fan doesn't run after the self diagnostic. The technician follows the Fan Check flowchart first.
- The fan runs but the compressor doesn't. The handbook has the technician confirm supply voltage (120V or 208V/240V depending on the model) at CN2 and CN3 on the daughter board, then voltage and connections at the compressor terminals, and replace the daughter board if no voltage reaches the compressor.
- The compressor runs a short time, or more than 5 minutes, and then faults. The flowchart checks the run capacitor's capacitance and the thermistor resistances against the handbook's table.
- Everything checks out and the compressor still won't run. A. O. Smith's flowchart says to contact Tech Support and that the water heater should be replaced. Check your warranty first.
- The handbook warns not to stand in front of the compressor terminals while testing, because they can dislodge and eject under pressure. That's a job for a technician.
Parts that commonly fix it
Always match the part number to your exact model.
A. O. Smith heat pump water heater air filter (exact part for your model)
A clogged filter starves the heat pump of air. Vacuuming it is free; replace it if it's torn.
Search eBay →Digital multimeter with capacitance setting
For a qualified person checking the run capacitor and thermistors, as the handbook's flowchart describes.
Search eBay →Heat pump water heater run capacitor (exact match for your model)
Technician part. Match the rating printed on the original, and discharge the old one before touching it.
Search eBay →Sources
- A. O. Smith Residential Hybrid Electric Heat Pump Water Heater Service Handbook (assets.hotwater.com): diagnostic codes 080, 083, 084 (Heat Pump Compressor is not functioning) and 085, beeper rule, HPO and Electric Mode, system diagnostic, Compressor Check flowchart pages 64 and 65
- A. O. Smith heat pump water heater Service Handbook for older models (assets.aosmith.com): ECC, Heat pump compressor is not functioning, and its compressor check flowchart
Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.