An induction cooktop does not heat a surface and let the pan sit on it. It drives a coil under the glass with an alternating current, and the pan itself becomes the heating element by absorbing that field, which only works if the base of the pan is magnetic and large enough to cover the coil. Samsung’s range and wall oven article lists the underlined u as a pan detection error on induction cooktops, with the instruction to ensure the cookware is suitable in size, material and placement. That is the whole diagnosis in most cases. The zone was asked to heat, found nothing it could heat, and said so.
Material
Press a fridge magnet against the pan base. A firm grip means induction will heat it. A weak grip, or none, means the pan is aluminum, copper, glass or a stainless steel that is not magnetic, and no setting on the cooktop will change that. Some pans have a magnetic disc bonded to a non-magnetic body; those work if the disc is intact and flat. A pan that worked on a previous induction cooktop is not proof, because detection thresholds differ between makers and a marginal pan can pass one and fail another.
Size and Placement
Every zone has a smallest usable pan size, given in the manual and often in the zone marking on the glass. A small saucepan on a large zone, a coffee pot, or a wok with a small flat base does not cover enough of the coil for the cooktop to detect it, even if it is magnetic. Center the pan on the zone marking. A pan that is off-center, or a pan with a warped base that only touches the glass at the rim, gives the same result. On Samsung cooktops and ranges with a flexible or bridged zone, make sure the mode selected matches how the pan is placed, since a pan straddling two zones in the wrong mode reads as not present on either.
When the Pan Is Right and the u Stays
Put a pan you know works on induction, magnetic, flat and correctly sized, on the zone and center it. If the u still shows, or if the zone heats briefly and drops out with the u returning, the problem is under the glass: the pan-sensing circuit on the power board, the coil, or a connection between them. With the cooktop switched off at the breaker, the glass is lifted and the boards are tested; that is a visit, not an owner task, because the power boards carry mains voltage and store energy after switch-off.
Other Induction Behavior Mistaken for a Problem
A zone that clicks or hums at high power with certain pans is the pan resonating in the field; it is normal and louder with lightweight or layered pans. A zone that switches itself off after a period is the auto-off timer, and a zone that drops to a lower setting on its own during a long boil is the cooktop managing the power shared between two zones on one board. A cooling fan running after the cooktop is off is cooling the electronics. None of these needs a visit. A crack in the glass, however small, does: with the cooktop switched off at the breaker, stop using every zone until the glass is replaced, because liquid through a crack reaches the boards beneath.
Booking
Give the model code from the label under the cooktop or on the frame of the range, say which zone shows the u and whether it happens with every pan or only some, and tell us if the zone has ever heated that pan before. Book a Samsung cooktop repair; if the same behavior is on a slide-in induction range, the range page is the right one, since the cooktop and oven share a control there.