Is a OBD P0420 causing a P0505?

I have a 1997 Honda Civic DX 1.6L and was wondering if my two check engine lights are caused by one another?

No, it is cause by two different sensors that was reported in your cars self diagnostics system.

Not one by another, but both by the same cause. P0420 is produced by the ECU when the O2 sensor for the exhaust after the catalytic converter reports a high and fluctuating free level of oxygen, while P0505 is produced when the idle air control valve is unable to control the idle speed (usually means the speed is too high). A large intake vacuum leak, especially in one branch of the intake manifold, will cause both.

In a 1997 Honda the intake manifold gasket is a very good suspect… It is paper and shatters. To test it, enlist a helper. Remove the duct from the air cleaner, plug the breather hole in it, and hold the throttle open. Put the duct to your face and try to pressurize it by blowing in it. There should be very little leakage. If you can exhale through it in a few seconds have your helper listen and feel along the gasket, top and bottom.

Yes this pair of codes… On the your engine is usually the coolant hoses clogged to The EACV or idle air control valve. These are the small hoses around the throttle valve area (they look like vacuum hoses). Or… You could have an air pocket in the cooling system… Or there's a plastic screw in some EACV that backs itself out and that causes the car to have an irradic idle, tighten it back down gently… Or the port for the idle air in frt of the throttle blade is blocked, use IAC cleaner it's in a can like carb or brake cleaner
Fix the P0505 first…