Mode 06: the car's own lab book
Manual 04 ended with the readiness panel: a list of self-checks, each either done or still running. Fair enough — but "done" is a strange word for an exam. Done how? Passed by a mile, or scraped through by a whisker? The car knows. Every time a self-check finishes, it writes the actual measurement into a page of results the standard calls mode 06 — what the catalyst check measured, what the misfire counter counted, and the exact window each number had to sit inside. It's the marked exam paper, kept in the glovebox of the engine computer, and almost nobody ever asks to see it.
what one line of it says
A mode 06 record is short and complete: which self-check, which test within it (one monitor often runs several), the value it measured, and the minimum and maximum it was allowed to be. That last pair is the whole philosophy. The car doesn't grade on vibes — it compares one number against two others, and the lamp you see on the dashboard is downstream of exactly this comparison. When the bay shows you catalyst: 900 in a window of 0–800 — FAIL, you're not reading an interpretation. You're reading the arithmetic the car itself did, the moment before it decided to store P0420.
why the numbers look like nonsense
Fair warning: mode 06 values arrive in the test's own internal units, and the standard's conversion table runs to a hundred-odd entries that cheap tools routinely get wrong. This page doesn't convert them at all, on purpose. Here's why that's honest rather than lazy: the value and its limits always arrive in the same units, whatever they are. Whether "900" means switching cycles, millivolts or something stranger, the only question that ever mattered is whether it sits inside 0–800 — and that comparison is exact no matter what the units are. It's precisely how the car judges itself. A number dressed up in confidently wrong units is worse than a raw one wearing none.
what it's actually good for
Two things, both quietly excellent. First: seeing the working behind a code you already have. A stored P0420 tells you the catalyst check failed; mode 06 tells you by how much. Failed at 900 against a ceiling of 800 is a very different conversation with a mechanic than failed at 2400 — one is a borderline case worth re-testing after a sensor check, the other is a cat with its bags packed.
Second, and better: catching the drift. Parts rarely fail like a light switch; they fade. A catalyst reading 780 against a ceiling of 800 is passing — readiness says done, no code, no lamp, nothing to see. But it's passing the way a student passes at 51%, and next year's exam is unlikely to go better. Glance at mode 06 once or twice a year and you see the number creeping toward its limit seasons before the lamp ever lights — which turns a breakdown-shaped surprise into a budgeted-for repair.
the small print, honestly
Three caveats, none of them secrets. Pre-CAN cars — most of the fleet before about 2008 — keep mode 06 in manufacturer-specific code with no public decoder ring, so this page declines to guess and says so on screen. Clearing codes erases mode 06 too — it's part of the same eraser stroke that wipes the freeze frame and resets the readiness monitors, one more reason the clear button deserves its stern confirmation box. And the results only exist once a check has run: a freshly cleared car, or one that's mostly done short cold trips, may honestly have nothing on file yet. The fix is the same boring week of mixed driving that finishes the readiness monitors.
ask for the lab book
Plug in, press test results in the bay, and read the marks. If everything passes with room to spare, enjoy the rare pleasure of documented good news. If something's failing, you now hold the number a fault code would only have hinted at. And if something's passing by a whisker — that's the one the lab book exists for. Note it on the job card, check it again in a few months, and let arithmetic do the worrying before the lamp has to.