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Thread: CAN OBDII - Can someone help me?

  1. #1
    Join Date
    Oct 2006

    CAN OBDII - Can someone help me?

    I am trying to develop a scan tool for my Brazilian Ford Fiesta that uses the CAN bus for diagnostics. I am using a PIC 18F4680 for that and I have been able to enter the bus and monitor its messages. However, when I try to send an OBD request to the car, say:
    ID: 0x7E0 (ECU #1)
    02 (ISO 15765 no. of data bytes)
    01 (mode 1)
    01 (PID 1)
    00 00 00 00 00 (Padded with zeros)

    I get the following error message:

    ID: 0x7E8
    7F (General response(error).... I should have received something like 41, right?)
    00 00 00 00

    Can someone shed some light on this? I have tried many different messages and I always get that disturbing 0x7F error code. Also, if I try ID 0x7DF I dont receive anything no matter what message I send. How can I make this work? Thank you.

  2. #2
    Low Bitrate
    Join Date
    Jul 2005
    $7F is indeed a negative response. A negative response frame is formated as such:

    ARB ID: $XXX, B1: $0X, B2: $7F, B3: $SS, B4: $NN, B5: $XX, B6: $XX, B7: $00, B8: $00
    Where $SS = Service Requested (in your case Mode $01) and
    Where $NN = Return Code.

    Here is a list of some return codes:
    $11 Service Not Supported
    $12 Sub Function Not Supported - Invalid Format
    $22 Conditions Not Correct Or Request Sequence Error
    $31 Request Out Of Range
    $35 Invalid Key
    $36 Exceed Number Of Attempts
    $37 Required Time Delay Not Expired

    It seems to me that your ECU does not support Mode $01. There is little you can do about this. You can try another ECU try $7E1-$7E7. Maybe one of them will work for you.

    Good luck.

  3. #3
    Join Date
    Oct 2006
    Thank you for your answer. It does turn out to be that my car is not OBD II compatible via the CAN bus, however, I can use the ISO 9141-2 interface it implements. Thank you once again.

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