A COMPUTER WEEKLY NEWS story (May 8, 1997) reports on chips that have code written on them. In 1996, 7 billion programmable microcontrollers were shipped. In 1995, it was 3.5 billion. As few as 5% are affected by y2k. The problem is, nobody knows which 5%.
There may not be enough replacement chips available.
Some are in inaccessible systems: communications satellites, undersea cables, buried pipelines, nuclear reactors, pacemakers, bank vaults.