The difference between Panasonic and Samsung/LG in this area is when Panasonic goes bad it tells you where the fault is and gives you good information on how to fix it. But with Samsung/LG you need to use test equipment and service guides to fix them.
The dealer would be lying by saying they "fixed" the problem, as displays continue to have faults with a low percentage today, and they continue to use the red light to indicate a fault condition where necessary. In my experience Panasonic plasmas are the most reliable of any plasmas, indeed of any TV, but they can and do go wrong sometimes. I read somewhere the defect rate (after a few years) is around 2~3% for Panasonic, 3~4% for LG and 6% for Samsung.