"Err" appears on the display when stepped on
The "Err" error code typically indicates an overload. Step off of the scale immediately and try removing the batteries from the scale for 90 seconds and then reinstall them to see if the scale will return to normal operation.

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Does not power on
Note: Brand new out of the box there is a plastic tab that will need to be pulled and removed from the battery compartment to activate the scale.
Have the old batteries replaced with a brand new set of AAA batteries. Ensure the batteries are installed properly with the positive and negative ends pointing in the right direction inside of the battery compartment.

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Scale does not weigh accurately
Ensure the scale is used on a hard, flat, stable surface, and not on carpet or rugs. If possible, try verifying weight with an exercise dumbbell greater than 11 pounds. The unit can be "calibrated" by turning it on (by pressing with your feet) and letting it power off one time before stepping on again. This will help clear any backend weight so that the scale does not register its own unit weight. There are also operating tolerances for bathroom scales; see below for ranges based on weight.

General bathroom scale accuracy/tolerance margins:

11 - 110 lbs: +/- 0.6 lb

111 - 220 lbs: +/- 0.8 lb

221 - 330 lbs: +/- 1.0 lb

331 - 400 lbs: +/- 1.4 lbs