2007 Cadillac CTS code 420

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

The rear oxygen sensor is switching between "lean" and "rich" too fast which indicates no change is taking place in the makeup of the exhaust gas. What's going in and being detected by the front sensor is the same as what's going out and being detected by the rear sensor. To determine that, both sensors have to be working. When they detect the same thing, it's because the catalytic converter isn't doing its job. That sets code 420.