Are any other codes present?
According to the 97 FSM, things to check for code P0420 are as follows:
1. Inspect wiring harness at PCM for damaged, backed out terminals and poor terminal and wire connections.
2. Inspect cat for dents, severe discoloration, holes, internal rattle
3. Inspect Exhaust system (from rear exhaust flange manifold to cat) for leaks, damage, or loose/missing hardware.
4. Inspect Rear Oxygen (H02S 2) sensor (right behind the cat) to insure the sensor is mounted securely, and that the electrical pig-tail and wiring is connected and is not contacting the exhaust pipe or damaged. Also check the connector at the end of the pig-tail for loose/broken connectors.