DTC P0420: 3-Way Catalyst Function: Procedure

Inspect exhaust system for damage and repair as necessary. Start engine and run at idle. Check for exhaust leak before three way catalyst. Check for intake air leak after MAF sensor. Repair or replace faulty components as required. If no leaks are detected, go to next step.
Turn ignition on. Using DVOM, check for battery voltage between PCM terminals No. 102 (Red/Black wire), No. 104 (Green/Black wire), No. 107 (Yellow/Black wire) and No. 109 (Blue/Black wire) and ground. See Figure. If battery voltage exists, go to next step. If battery voltage does not exist, repair open or short in appropriate wire(s) as required.
Disconnect ignition wires from spark plugs individually and connect a known good spark plug to wire. Place spark plug on good ground. Crank engine and check for spark. If spark exists at every plug wire, go to next step. If spark does not exist, check and repair spark plug wires, ignition coil and power transistor. See SYSTEM/COMPONENT TESTS .
Turn ignition off. Remove injector assembly. Keep all associated fuel lines connected. Disconnect CMP sensor harness connector. Turn ignition on. Verify fuel is not dripping from injectors. Replace injectors which are leaking. If no injectors are faulty, go to next step.
Check ignition timing and adjust as necessary. See ADJUSTMENTS article. Check PCM pins and terminals for damage and check connection of PCM harness connector. Reconnect PCM harness connector and retest. If DTC resets, replace three way catalyst.