The cost of repairing the P0420 error code can vary depending on the cause. If you need to replace the catalytic converter, the cost can range from $500 to $2000. If you need to replace the oxygen sensor, the cost can range from $100 to $200. The cost of repairing a leak in the exhaust system will vary according to the amount of damage. A minor repair might cost $100, while a more extensive repair might cost $1000 or more. Ultimately, the cost of repairing the P0420 error code will depend on the cause of the problem.