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Thread: OBDII DTC processing

  1. #1
    Fuel Injected!
    Join Date
    Jan 2019
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    OBDII DTC processing

    Post edit...

    I came across a description of four different types of dtc. Description is a bit fuzzy but from what I read, types are:

    Type A
    -Emissions related.
    -Requests illumination of the MIL after one failed driving cycle.
    -Stores a freeze frame DTC after one failed driving cycle.

    Type B
    -Emissions related.
    -Sets a Pending Trouble Code after one failed driving cycle.
    -Clears a Pending Trouble Code after one successful driving cycle.
    -Turns on the MIL after two consecutive failed driving cycles.
    -Stores a freeze frame after two consecutive failed driving cycles.

    Type C
    - Non-emissions related.
    - Type C codes can cause the MIL lamp to come on (or illuminate another warning lamp)

    Type D
    - Non-emissions related.
    - Type D codes do not cause the MIL lamp to come on.


    In the code I discovered three different "enable" tables. I believe they relate to the types. Not four tables because type d is flagged only on a code reader.
    Perhaps someone with knowledge could make all this clear for us. This relates to tuning in a number of ways. Control of various thresholds, re-calibration of
    replacement sensors that are modified from the manufacturers and also finding parameters within the code.

    End edit

    Hi,

    I hope someone can help me out with details of how DTC codes are created and processed by GM ECMs. The ECM I am working with is one of the first OBDIIs but I expect GM always handles DTCs the same way. I am working my way through a sub routine and this is the flow I see...

    - A pointer is set up to a lookup table that formats bit position, byte offset and DTC codes.
    - A FLASH table is looked at to see if the DTC is enabled. (is enabled the right term or is there GM speak for this?)
    - First RAM table is looked at to see if the DTC has been detected. (is detected the right term?)
    - Second RAM table is looked at if the DTC is already set in table 1. (is confirmed the right term?)
    - Third table is looked at if the preceding tables are set. ( is pending the right term?)
    - Specific DTCs relating to misfire and fuel trim are singled out for special process.
    - Fourth table is looked at and if clear, the bit for this DTC is set in this table only.
    - Fifth table is looked at if table 4 is set.

    The code loops through all 18 bytes for each bit through each table. I assume all this is to filter transient conditions such that only "real" DTCs remain are flagged.

    I would appreciate help getting the terminology right so that when folks look at the comments there is no need to translate into more common terms.

    Thanks -Tom
    Last edited by Tom H; 02-10-2019 at 12:41 PM.

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