DTC P0420: 3-Way Catalyst Function: Procedure

Inspect exhaust system for damage and repair as necessary. Start engine and let it idle. Check for exhaust leak in front of 3-way catalyst. Check for intake air leak after MAF sensor. Repair or replace faulty components as required and retest system. If no leaks are detected, go to next step.

CAUTION: DO NOT use ECM ground terminals when measuring voltage.

Turn engine off and turn ignition on. Measure voltage between ground and ECM terminals No. 102, 104, 107 and 109 by backprobing. See Figure. If battery voltage exists at all terminals, go to next step. If battery voltage does not exist at all terminals, repair appropriate injector or circuit and retest system. See SYSTEM & COMPONENT TESTING article.
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 every plug wire, repair ignition system as necessary and retest system. See SYSTEM & COMPONENT TESTING article.
Check ignition timing. See ON-VEHICLE ADJUSTMENTS article. Adjust as necessary. If ignition timing is okay, 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.
No problem is indicated at this time. Problem may be intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article. If DTC resets, replace three-way catalyst.