How old is your van?
If it’s already several years old, one or more of your tire pressure sensors might already be dead, as in dead batteries inside the sensors. That would explain why correcting the tire pressures did not result in the warning light extinguishing
P0420 is a code signaling that the catalytic converter efficiency is too low, as far as the powertrain control module is concerned. Proper diagnosis is required, though.
As a matter of fact, “generator” is the technically correct term. I’m not being a smart alec. Several years ago, it was decided that the word "generator was to be used again, at least in technical literature
Please click on the link. There’s some moderately interesting information contained within