Highway driving does not damage the cat.
There are several causes of the P0420 error. The most common, and most
likely in your case, is simply a failing cat. Some models (not yours) had
problems with poor connections at the igniter, which the ECM misrecognized
as a failing cat. An exhaust leak after the primary O2 sensor is another
possible cause, as is a cracked exhaust manifold.
Honda has a TSB for your car for this specific problem (03-073). The
speicifed cure is cat replacement.
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