A. O. Smith · Voltex hybrid with letter codes (HPTU, 2016–2019)

A. O. Smith Voltex hybrid with letter codes (HPTU, 2016–2019) error code E20 or E21

Upper temperature sensor

What it means

Upper Temperature Sensor is not functioning.

How it shows: may have an audible alarm; to cancel the audible alarm, press any button on the UIM

Source: Hybrid Electric Heat Pump Water Heater Installation Instructions and Use & Care Guide, 100323093_2000578009, page 20

What A. O. Smith says to do

These steps involve electrical testing or parts replacement. A. O. Smith's manuals tell owners to leave that work to a qualified service technician.

Power off (Power Cycling the unit), disconnect and reconnect J2 and J5 connections from control board, inspect wire for damage, re-install UIM, reconnect power. If error persists, replace temperature sensor.

Last checked 2026-10-03.

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)

From the manufacturer's manual for this series.

The same code on other models

Codes mean different things on different heaters. Match the series to the model number on your rating label.

A. O. Smith Voltex hybrid with letter codes (HPTU, 2016–2019) error code E20 or E21

What it means

What A. O. Smith says to do

Safety first

The same code on other models

Other Voltex hybrid with letter codes (HPTU, 2016–2019) codes