Why is it sooo hard to find information about mode $06 (mode six, mode 6)? Almost every hit on google is some guy selling his "secret" information that'll "unleash the power of OBDII Mode $06!!!"

Well, after poking around a bit, I've figured it out I think. I got an OBD2 All-in-one scanner (ELM327 based) recently, and have been playing around with it. This lets you manually send data to the OBDII bus. Using this and the Honda mode $06 documents from https://techinfo.honda.com/rjanisis/RJAAI001_mode.asp, I was able to determine the following.

This is what I sent in hyperterminal (this is all in hexadecimal):

06 - indicate this is a mode $06 requestCode:`06 21`

21 - request parameter identification (PID) code 21

(and for mode 6, PID is equivalent to test identification (TID))

The response:

4 - indicates this is a responseCode:`46 21 81 00 09 00 04`

6 - to a mode 6 request

21 - for PID 21 (i.e. TID 21)

81 - indicates this is a result from component ID (CID) 1, and the test limit here is a lower limit

00 - padding? used for larger values?

09 - test value

00 - padding? used for larger values?

04 - test limit

One value needs more explanation, that value 81. This is one byte, and can range from $00 to $ff. The first bit, or most significant bit (MSB), indicates whether the test limit given is a lower limit (bit=1) or upper limit (bit=0). The lower 7 bits indicate the component ID (CID). So in this case, $81=0b10000001 in binary, the top bit is 1 so this limit is a lower limit, and the lower 7 bits indicate this is a CID of 1.

So, we open up the mode 6 doc for the 1998 Honda Civic, and you find the following:

For a TID of $21 and CID of $01, they tell you the following:

So our test result was 9, which when multiplied by their conversion factor gives 4.394529 mmHg, compared to a lower limit of 1.953124 mmHg. Which means, we passed the test, which is why the CEL/MIL lamp hasn't gone off. If the value reported was below the lower limit, then according to the mode 6 doc, a P1456 code would be issued and the CEL/MIL lamp would come on.Quote:

Monitoring the fluctuation of fuel tank pressure sensor output before and after

EVAP bypass solenoid value is opened after engine starting with cold condition.

Measured value: Output value (Decimal) x 0.488281 (mmHg)

The lowest limit value: Output value (Decimal) x 0.488281 (mmHg)

So, what do you all think of this? Is this right? It only took me an hour or so of tinkering around to figure it out. Why is it made to be such a secret?

If you look at a more recent model year, you'll find there are A LOT more tests that you can lookup. The mode 6 doc for the 2006 Hondas is 37 pages long, with all kinds of different tests, like finding out the number of times a cylinder misfired.