Snap a photo — see what it is, how urgent, who to call.
One photo of the problem. fixRAgent reads it and gives you a Triage Profile.
Triage a photo nowNo sign-up.
Usually not an emergency. If it smells like burning, sparks, or trips the breaker, switch it off at the breaker and leave it off until it is checked.
Take the photo. We’ll tell you what it is.
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Managing this for someone else? Managers: set it up for your buildings →
Managing this for someone else? There is a version of this for whole portfolios.
Managers: set it up for your buildings →“Lower thermistor sensor failure”
— A.O. Smith, A.O. Smith Hybrid Electric Heat Pump Water Heater Service Handbook. Codes vary by model: check your heat pump water heater’s own manual.
In plain words, 004 is about a temperature sensor or its wiring — the control reads a value it does not trust, or a temperature outside its limits.
A.O. Smith gives 6 other temperature sensor codes for this heat pump water heater: 003 (Upper thermistor sensor failure), 025 (Heat Pump Coil Temperature Sensor), 026 (Heat Pump Suction Temperature Sensor), 027 (Heat Pump Discharge Temperature Sensor), 028 (Ambient Temperature Sensor failure).
Next to 004 in A.O. Smith’s table: 003 (Upper thermistor sensor failure) and 006 (Internal Processor Error).
Safety: If it smells like burning, sparks, or trips the breaker, switch it off at the breaker and leave it off until it is checked.
At least THIS WEEK. fixRAgent treats this code as a this-week repair: the appliance has reported a fault, but the code alone does not point at a safety hazard. This is fixRAgent’s reading, not the manufacturer’s. A photo can raise it — water near wiring, scorch marks, a gas smell — but a code never lowers what the photo shows.
Plumber. Have the model and serial numbers from the label ready — a photo of the label works.
Check whether your model is under a safety recall on the U.S. Consumer Product Safety Commission’s site: cpsc.gov/Recalls. Your owner’s manual is on A.O. Smith’s support site; search it with the model number from the label.