Lin Bus - 00256 - A/C Pressure/Temperature Sensor (G395)

[gdallas]

Hi folks,

I’m trying to diagnose the source of the fault that VCDS is complaining about regards my G395 pressure sender;
00256 - A/C Pressure/Temperature Sensor (G395)
What I have done to date:

• I have replaced the part with another OEM. But fault persists and instantly annunciates its self following a clear of DTC through VCDS.
• I have traced the cables back visually as best I could given the current flow diagrams. Could not test continuity with built in DVM buzzer; presumably because the impedance was too great on the Lin Bus. But visually I see no defects or issues from the G395 sender > A pillar coupling station > A pillar coupling station > J519 module.
• With the sender disconnected, I have verified a healthy 14V supply at the wiring loom connector into the G395 sender, similarly the ground is also good.
• I have no means to test the validity of the data coming from the 3^rd a final core, I.e. the Lin Bus, but what I can say is there is no short to gnd. With the sender disconnected I am able to measure at the connector, a voltage which fluctuates from approximately 4.5 to 6 V with respect to gnd. My DVM has a frequency measurement option, and so I had expected to be able to measure ‘something’ from the master (J519), on the assumption it would be continuously issuing commands on the bus. Unexpectedly (to me), my DVM measured 0HZ, so only DC was present. I am not sure if this is expected or if my DVM should have been reading the clocked messages, or would there be no clocked messages if no devices were on the bus? Im just not sure. I don’t believe there is anything else on this particular bus, as I don’t appear to have a G238 sender on my vehicle for example. Is there an idle DC voltage when all slaves are disconnected? I have reviewed the message format of Lin Bus with respect to command and response, but I’m not sure if the master keeps issuing messages hoping for a response eventually, with no slaves attached or if it goes into another state where only a reference DC signal is present, awaiting a slave to momentarily take it to gnd, as a heads up to the master to allow the master to then start issuing commands?!

I’m kind of at a brick wall and not sure what my next move should be. It appears that each of my 3 pins at the connector are doing what they are supposed to be (notwithstanding my uncertainty with pin 2 (Lin bus), so is my new OEM G395 which I bought from Audi last week, just defective?
Any other routes of enquiry would be greatly appreciated.
Thanks guys

[Crasher]

What is the exact definition of the code? Have you checked it has adequate gas pressure?

[gdallas]

What is the exact definition of the code? Have you checked it has adequate gas pressure?

Hi Crasher,

The fault is

1 Fault Found:
00256 - A/C Pressure/Temperature Sensor (G395)
004 - No Signal/Communication
Freeze Frame:
Fault Status: 01100100
Fault Priority: 3
Fault Frequency: 1
Reset counter: 57
Mileage: 42778 km
Time Indication: 0
Date: 2018.12.16
Time: 12:44:15
I did take it to Halfords whom performed a charge pressure test; however the chap who performed this admitted he didn’t really know what he was doing. The can of refrigerant with pressure gauge he hooked up appeared to show a pressure in the green section, so I assumed it was ok! But who knows, perhaps worthwhile getting it checked somewhere else too. But even with that all said and done, the measured values for G395 in module 9 shows a value of “-“without quotes. Which to me, suggests that nothing is being read, rather than a low pressure reading.
I did mention in my first post that I didn’t appear to have a G238, but that was incorrect. Last night I pulled the fuse that I knew fed the G395 and fresh air intake sensor(s), and low and behold, a Humidity sensor alarm appeared too. So I either have a combined G695/G238, or just a G695; according to the repair manual. Which I am kind of glad about to be honest, as since that sensor is electrically powered from the same source, and coupled to the same LIN bus as G395 (see link below), it means I can unplug G695/G238 from the harness and then interrogate the LIN with from my DVM to see if it checks out with what I seen on the G395 plug. If it does, then I will wire (slight variation on pin assignment) the G695/G238 into the plug used for the G395 to verify the integrity of the LIN bus wire there. If that all checks out and the G695/G238 is still operating as expected, then I will deduce that the replacement G395 is faulty, else the Lin wire is not right! I unfortunately do not have an oscilloscope to verify Lin Bus comms.
Hope to do this later on this afternoon if it’s not raining. I don’t want to be exposing the humidity sensor to any rain!

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