HF1 - DTC 0420 or 0430: Check For Misfire Monitor DTCs

*** THIS ARTICLE REFLECTS CHANGES MADE BY TSB # 9326B7, December 1993

Diagnostic Trouble Code (DTC) 0420 indicates that Bank 1 catalyst system efficiency is below the acceptable threshold.

DTC 0430 indicates that Bank 2 catalyst system efficiency is below the acceptable threshold.

NOTE 1:
Complete the spark timing check in Quick Test before proceeding with this test step. Spark timing retarded below specification may increase exhaust gas temperature and decrease catalyst efficiency over time.

NOTE 2:
Be sure customer has not:
- Refueled vehicle with leaded gasoline.
- Experienced high vehicle oil consumption.

NOTE 3:
If entering this Pinpoint Test for symptoms only, go immediately to HF6. HF6 - Check Fuel Pressure

NOTE 4:
Internal deterioration of a catalytic converter is usually caused by abnormal engine operation upstream of the catalyst. Events that may produce higher than normal temperatures In the catalyst are particularly suspect. For example, misfiring can cause higher than normal catalyst operating temperatures.

Possible Causes: (Catalyst and Exhaust System Concerns.)
- Use of leaded fuel.
- Oil contamination.
- Cylinder misfiring.
- Damaged HO2S.
- Damaged ECT sensor.
- Downstream HO2S wires improperly connected.
- Fuel pressure too high.
- Damaged exhaust system pipe.
- Damaged exhaust manifold.
- Damaged muffler/tail-pipe assembly.
- Damaged catalytic converter.

- Key On. Engine Off.

- Retrieve and record all Continuous Memory DTCs (MIL and non-MIL).

- Were any of the following misfire monitor DTCs recorded: 0300, 0301, 0302, 0303, 0304, 0305, 0306, 0307 and 0308?

YES -- GO to Powertrain Diagnostic Trouble Code (DTC) Charts to address the misfire monitor DTCs present. [1][2]Diagnostic Trouble Code (DTC) Charts

NO -- GO to HF2. HF2 - Check For HO2S Monitor DTCs