Using Mode 6 to Catch Failing Components

A trouble code means the failure has already happened. The component has crossed the threshold and the computer flagged it. But what about the component that is on its way to failing — passing today but barely? Mode 6 catches those. It shows you the test results the computer runs in the background, complete with measured values and pass/fail thresholds. A component trending toward the failure limit is a component you can address before the customer comes back with a check engine light and a bad experience.

Connect your scan tool. Go to the OBD-II generic or global menu — not the manufacturer-specific menu. Find Mode 6 or On-Board Monitoring Test Results. You will see a list of Test IDs. Each one has a test description (on better scan tools), a measured value, a minimum limit, and a maximum limit. If your scan tool only shows TID numbers without descriptions, you need to cross-reference with manufacturer documentation to know what each test measures. It takes extra effort but the information is invaluable.

Look at each test result and compare the measured value to the pass/fail threshold. A test that passes with a huge margin is healthy — no concern. A test that passes but the measured value is within 10 to 15 percent of the failure threshold is trending. That component is degrading. Catalyst efficiency is the most common example. The test compares rear O2 sensor activity to front O2 sensor activity. A healthy converter shows a rear O2 that barely moves. As the converter degrades, the rear O2 starts mimicking the front O2 more and more. Mode 6 shows that ratio climbing toward the P0420 threshold months before the code sets.

Misfire counters — Mode 6 tracks misfires per cylinder over a defined number of combustion events. Zero is perfect. A low count that has not hit the misfire code threshold means that cylinder has an emerging problem. Check the spark plug, coil, and injector for that cylinder before the misfire becomes constant. O2 sensor response time — the test measures how fast the sensor switches from lean to rich. A sensor that passes but is close to the maximum allowed response time is getting sluggish. Recommend replacement at the next service. EVAP system leak test — the test measures pressure decay in the fuel tank vapor system. A result close to the leak detection threshold means a seal is starting to fail — a gas cap, a purge valve, or a vent valve is degrading.

When you find a component trending toward failure, document it. Print the Mode 6 data or photograph the screen. Show the customer the measured value versus the threshold. Explain that the component passes today but will fail within a predictable timeframe. This is proactive service — not upselling. You are saving the customer a breakdown, a tow, and a more expensive repair later. The tech who uses Mode 6 regularly builds customer trust because the predictions come true. The converter you warned about three months ago just set the code — and the customer remembers you called it.