Q: Code 420 on 98 Corolla.

If the converter is functioning, the voltage output from the downstream oxygen sensor should be somewhat steady at around half a volt. What you seeing in the car with the P0420 code is a high-low voltage signal from the downstream oxygen sensor that mirrors that from the upstream sensor and that mirroring simply reflects the fact that the catalytic converter is doing nothing. If the converter were using the available oxygen in the exhaust stream in any oxidative reactions, the downstream sensor output would hold steady at around 0.5 volts. Instead, at present, what comes in, as reflected by the upstream sensor, is exactly what leaves, as reflected by the downstream sensor. Use an infrared thermometer to measure the converter inlet temperature versus the outlet. If the two measurements are identical, that will confirm that the converter is dead. If you desire additional confirmation and a diagnostic by a certified Mechanic, dispatched by YourMechanic right to your location, please request a check engine light diagnostic - code P0420 and the responding certified mechanic will get this resolved for you, including converter replacement if warranted and if desired. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.