Tech Tip: Using Mode $05 to Diagnose P0420

Mode $05 tells you how fast the O2 sensor is switching, a P0420 code is set along with pending code
P0420 Catalyst System Efficiency Below Threshold Bank 1. The computer runs a test switching lean to
rich and rich to lean (remember the computer can add fuel quicker than it can take it away) to see how
long the switching times are. We like to see times in the 22 to 28 MS range no more than 40 MS.

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Fuel Trim Diagnostics

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In this program Jim Wilson shows you how to use fuel trim as a diagnostic tool to test certain sensors and to verify your auto repairs. He explains how things like high mileage, dirty injectors, lazy O2 sensors or coking affect short term fuel trim. He also shows you how and what sensors affect fuel trim and how the catalytic converter is involved.

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