DTC P0420: 3-Way Catalyst Function (Except CALIFORNIA & I30): Procedure

Visually inspect exhaust system for damage or leaks. Start engine and run at idle. Check for exhaust leak in front of warm-up 3-way catalyst. Check for intake air leak downstream of MAF sensor. Repair or replace faulty components as necessary and retest system. If no problems exist, go to next step.
Check ignition timing. See ON-VEHICLE ADJUSTMENTS article. Adjust, if necessary and retest system. If ignition timing is okay, go to next step.
Turn engine off and turn ignition on. Measure voltage between ground and ECM terminals No. 101, 102, 103, 104, 105 and 107 by backprobing. See Figure and Figure . If battery voltage exists at each terminal, go to next step. If battery voltage does not exist at any terminal, repair fault in appropriate circuit and retest system. See WIRING DIAGRAMS article.
Turn ignition off. Remove ignition coils from rocker covers. Connect a known-good spark plug to coil and ground spark plug end. Crank engine and check for spark. If spark exists at all coils, go to next step. If spark does not exist at any coil, diagnose fault in coil or coil circuit and retest system. See DTC P1320: IGNITION SIGNAL .
Turn ignition off. Remove injector assembly, keeping fuel hose and all injectors connected. Disconnect CMP sensor harness connector. See Figure. Turn ignition on. Verify fuel is not dripping from any injectors. Replace any injectors from which fuel is dripping and retest system. If all injectors are okay, go to next step.
Problem may be intermittent. Attempt to recreate conditions present when DTC was set. See TESTING PROCEDURE under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article. If no problems are found, replace warm-up 3-way catalyst and retest system.