Battery first: CR2032, CR2025 and the cells that look alike
The number on a coin cell is its size. A CR2032 is 20 millimeters across and 3.2 millimeters thick; a CR2025 has the same width at 2.5 millimeters. Both fit the same slot, but the thinner cell can sit loose in a fob built for the thicker one and lose contact when it is bumped. Match the number on the old cell.
Many push-to-start cars show a low key battery message on the dash before the fob quits. If range stays short after a fresh cell, the fault is in the fob or the car, and that is worth a call.
Buying a fob online: read the back before you pay
Every fob sold in the US carries an FCC ID, and the maker's part number is on the case or the board. Match both to the fob you are replacing, not just its shape. Two fobs can look identical and transmit on different frequencies, 315 MHz on most US cars and 433 MHz on some others, and the car listens on only one.
Toyota adds a wrinkle. Some Toyota remote head keys kept one FCC ID across chip generations, and the letter stamped on the blade near the head, G or H, tells them apart. The wrong chip will lock and unlock the doors and never start the engine. A photo of your key settles it.
Why a salvage-yard fob usually will not pair
On most makes, pairing writes that car's data into the fob's chip and locks it there. A used fob from a junkyard or a secondhand listing will then refuse a second car, even with a matching FCC ID and part number. A special tool can reset some locked fobs, but whether it works depends on the exact part.
That is why the fob you buy should be new, or at least never paired. A listing marked used or refurbished needs a straight answer from the seller on whether the chip was ever programmed, and no answer means it was.
What re-sync means, and why pairing needs the car
Remotes use a rolling code. Each press sends a new number, and the car accepts only numbers inside a window ahead of the last one it heard. A fob pressed dozens of times out of range, in a pocket or by a child, can run past that window, and a battery swap can knock the count loose. Re-sync is the short routine that lines the two counts up again.
Pairing is a different job. The car stores the ID of every fob it will answer, so a fob can only be added with the car there. Some older cars let owners do it with an ignition-and-button routine from the manual; most newer ones need a programmer on the diagnostic port and security access to the car.