The post cat O2 sensor should read low. The converter oxygenates the exhaust, adding oxygen, and this drives the oxygen sensor voltage low (high oxygen = low voltage). This is what the computer is looking for: is the post cat O2 sensor voltage consistently lower than the pre cat sensor? This is something we can't determine by watching the values on a scan tool or even a scope, the computer has a calibration that varies depending on any number of operating conditions, and the thresholds are programmed into the software. We don't know what the thresholds are, and it wouldn't matter if we did, because we can't monitor and measure it fast enough with our naked eye anyway. We rely on the engine
computer to do this math for us, and it does it very reliably. If the shop only charged you $270 to replace the converter, then it's not the correct converter. Half of that money was likely labor, which means the converter was about $135, and they made money on that converter, which means they probably paid $50 for the converter itself. $50 converters don't meet the emissions standard, and will turn the light on and you'll have a P0420. This is the reality of it. The correct converter for this car will cost much more, the part along will be $400-600, plus labor. If you're not paying that much for a converter, you're not getting the converter you need. You can't go cheap on this, the computer will not be satisfied, and you'll never get P0420 to go away. A "new" converter is not a "good" converter. Don't get hung up on the fact that it's new, that doesn't mean anything. Other makes and models (VW, for example) require a converter that's about $1000. There are companies that make cheaper replacements, but if you don't put the expensive (that is, correct) converter in, P0420 will plague you until you do. This converter is just wrong.