In the way of background info, your car computer (the ECM) constantly checks the operation of the cat converter while the car is being driven. It has sensors before and after the Cat, and if the cat is working, the sensor readings confirm this to the ECM. And if the Cat isn’t working, the sensors say this to the ECM too, and if they keep saying the Cat isn’t working, the ECM will turn on the check engine light.
When your new cat was installed, the Cat shop probably reset the ECM. Or they disconnected the battery, which does the same thing. After being reset, the ECM has to read those Cat sensors over the course of several trips, and during different operating conditions. Low speed, freeway speed, low coolant temps, high coolant temps. It’s not simply driving a certain number of miles, the car has to go through all these various prescribed driving conditions before it will finish the ECM reset procedure. So if your 100 miles of driving were all more or less the same type of driving conditions, then it might not reset. It’s a shame the shop isn’t able to tell you what exactly you need to do to finish the reset procedure. There ought to be a regulation for the auto makers about this in my opinion, that the ECM can’t keep secrets from the driver. And I think the smog shop should offer you better guidance too, rather than simply telling you to drive more and come back later.
You might wonder how the Smog shop even knows about all this? When you go to the smog place the first thing they do is hook up their computer to your car’s ECM and see what the ECM has to say. If the ECM says it has recently been reset, your ECM tells this to the smog shop computer. Your ECM in other words tattles on you. This is to prevent the circumvention of the state’s smog laws by simply disconnecting the battery before going for the test.