Error code P0420 is not going to cause an immediate risk to your safety, however, it’s generally not advisable to continue driving a vehicle after you discover a P0420 code. If there is a problem with the exhaust system then this will only cause more damage over time. A faulty oxygen sensor is a relatively cheap repair, but left unfixed it can lead to damage to the catalytic converter that may end up costing $1000’s to put right.