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With the 2005 Kia Cerato I did there was 2 immo fault codes in ECM & with testing it looked like the antenna (95420-2D100) was part of the immobiliser unit & looked to have circuitry that may have needed programming, but bought a 2nd hand unit (most wreckers wouldn't separate from lock set controllers kit, but found a reasonably priced unit, brand new from Kia was around $290 or from Hyundai $460) & fixed without programming. When taking cover off unit the 2 wires for coil around barrel (assuming this is the antenna) had arcing marks around where coil wires join to pins & guessing that’s the main issue.

Hopefully this may help any others out there who come across it with Hyundai Elantra or Kia Cerato.