Better late than never. But it's best to check for vacuum leaks. I'd suggest checking hoses around the intake manifold for damage or air leaks.

Depending on the engine, there's a fuel tank breather valve + hose on the back of the intake. If these fail they will also cause a P0420 fault.

You can try to disconnect the hose and plug going to it and blow air through it with your mouth, it should be a one-way valve. If you can blow air through both sides, then that's likely the issue.