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If a valve broke more than likely you will need a new engine, at least you need a piston and anew cylinder head and camshaft. Now for some odd reason if only 1 valve broke and no engine damage occurred then absolutely no they should not have to replace the other valve nor should you. Either way they are responsible for parts and labor unless they prove it had nothing to do with the repair they made which would be highly unlikely. Sound like a bad part was installed or a poor job was done. Please give a positive rating. Thanks