OBD II is constantly sending " 7F 01 12"

Question

I am writing a program that gets the speed and fuel rate of a car from the OBD II computer. Getting the speed works fine, but I always get "7F 01 12" when asking for the fuel rate. How can I fix this?

I am using this to get the data from the OBD, and here is my code

main.py:

from OBD import OBD
import datetime

f = open('log.txt', 'w')
obd = OBD()

while True:
    #Put the current data and time at the beginning of each section
    f.write(str(datetime.datetime.now()))
    #print the received data to the console and save it to the file
    data = obd.get(obd.SPEED)
    print(data)
    f.write(str(data) + "\n")

    data = obd.get(obd.FUEL_RATE)
    print(data)
    f.write(str(data) + "\n")

    f.flush()#Call flush to finish writing to the file

OBD.py

import socket
import time

class OBD:
    def __init__(self):
        #Create the variables to deal with the PIDs
    self._PIDs = [b"010D\r", b"015E\r"]
    self.SPEED = 0
    self.FUEL_RATE = 1

    #Create the socket and connect to the OBD device
    self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    self.sock.connect(("192.168.0.10", 35000))

def get(self, pid):
    if pid < 0 or pid > 1:
        return 0

    #Send the request for the data
    if self.sock.send(self._PIDs[pid]) <= 0:
        print("Failed to send the data")

    #wait 1 second for the data
    time.sleep(0.75)

    #receive the returning data
    msg = ""
    msg = self.sock.recv(64)
    if msg == "":
        print("Failed to receive the data")
        return 0

    print(msg)

    #Process the msg depending on which PID it is from
    if pid == self.SPEED:
        #Get the relevant data from the message and cast it to an int
        try:
            A = int(msg[11:13], 16)#The parameters for this function is the hex string and the base it is in
        except ValueError:
            A = 0

        #Convert the speed from Km/hr to mi/hr
        A = A*0.621
        returnVal = A
    elif pid == self.FUEL_RATE:
        A = msg[11:13]
        returnVal = A

    return returnVal

Thank you!

Answer 1

This won't be a direct answer, because this issue is hard to troubleshoot without replica of the car. A 7F-response is a negative acknowledge.

So it could be that the model/make doesn't support that PID. You can check that by send a query. Because fuel rate is requested by sending '015E', you will have to request the '0140'. This will return a bit-encoded answer, which you can parse to know if your inner OBD-II bus supports your '5E' pid.

To decode the bit-encoded answer, check this link: http://en.wikipedia.org/wiki/OBD-II_PIDs#Mode_1_PID_00

If '5E' is not supported, that's the answer on your question. If it is supported, there is something else wrong.

EDIT: Just found out that 7F 01 12 means the PID is not supported. But you can try to double-check with the bit-encoding. https://www.scantool.net/forum/index.php?topic=6619.0

Answer 2

0x7F represents negative response.

0x01 is the function you attempted

0x12 represents sub-functionNotSupported (according to many ISO documents)

Some other common error codes are

 0x13 incorrectMessageLengthOrInvalidFormat
 0x22 conditionsNotCorrect
 0x33 securityAccessDenied

You may need to enter a DiagnosticSession in order to get some functions to become "supported"