- P0420 – catalyst system efficiency below threshold. Found very minimal RPM drop during EGR system test. Removed EGR pipe to intake manifold and found it was 75% restricted. Cleaned carbon out of pipe. RPM now has severe drop (nearly stalls) during EGR system test as it should. This should help with high NOX readings. Catalytic converter is bad as well. Upstream and downstream 02 sensor readings are nearly identical. Need to replace catalytic converter.
Bob W. gave our service a 5 star review on 8/30/2022