Customer Concern: Setting trouble codes P0420 and P0430.
Tests/Procedures: 1. Use a scan tool to monitor all 4 oxygen sensors while driving the car at a steady cruise.
2. With the vehicle warmed up at a steady speed, the upstream sensors (HO2S11, 21) should continue to switch rich to lean very rapidly. The downstream sensors (HO2S12, 22) should actually start to show very little switching rich to lean. If the downstream sensors continue to switch similar to the front sensors, replace the worn out catalytic converter(s).
3. Before replacing catalytic converters, be sure there are no misfire problems, no fuel control problems, and no problems with engine oil or coolant usage that could damage a replacement catalytic converter. Also verify there are no exhaust leaks or modifications that would cause any of the sensors to read incorrectly.
Tech Tips: When replacing the catalytic converters, use Original Equipment Manufacturer (OEM) parts or parts that guarantee to meet or exceed OEM specification. In some cases, replacement catalytic converters can clean up tail pipe emissions but may not satisfy the PCM monitor for them, causing the catalyst efficiency codes to return.