[BNaylor]

You are not that far off in NOx readings. The CAT only works efficiently when it is up to operating temperature so make sure your car is fully warmed up before going back in for the emissions test.

The CAT convertor would be the next likely choice. The first stage is called the reduction catalyst which reduces NOx. Other possibility since it is running lean would be to try a new 02 sensor. Its important to ensure fuel injection system is at 14.7:1 air/fuel mixture.

$450 is too expensive for a CAT and labor, even a OEM GM. You should consider going with an aftermarket obd-i/ii compliant CAT convertor which should be a lot less. Good luck!