New Permatex Gasket Maker.  This is Ultra Copper, and is a maxium temperature sealer.  This is sensor safe, and works for applications from 65 deg F to 700 Deg F.  We typically use Copper sealer for sealing exhaust flanges.

TIPS : Not all gasket sealers are the same.  The color of the sealer is typically for a specific fluid or application, although some are multipurpose.  Using the right sealer works properly, using the wrong sealer may cause premature leaks.