The information I found about this particular code is as follows: Code: ECM-P042000 Definition: Catalyst System Efficiency Below Threshold (Bank 1) Description: Engine Control Module (ECM) checks the efficiency of catalytic converter (bank 1) by switching between rich and lean fuel/air mixture. When the change-over is made, the signal from the rear heated oxygen sensor (HO2S) is checked and the time taken for the probe to switch. The diagnostic trouble code (DTC) is stored if the engine control module (ECM) registers that the rear heated oxygen sensor (HO2S) switches too quickly and is interpreted as poor catalytic converter efficiency. The diagnostic trouble code (DTC) can be diagnosed with the engine at operating temperature, approximately 15 minutes after the engine has first been started if the vehicle is driven (normal road load) at 70-90 km/h for 30-40 seconds. Cause: Air leak in the exhaust system. Damaged front heated oxygen sensor (bank 1). Damaged front catalytic converter (bank 1).
