2008 Audi A6 No AC 00256

Brian Owner Parma, Ohio Posted   Latest   Edited  
Updated
Driveability
2008 Audi A6 3.2L (BKH) (01J)
00256
No Compressor
No A/C

Hello All , I am weak on German vehicles and I may be making a obvious over site.

This A6 had a aftermarket pressure switch installed due to no no compressor operation or cooling. I arrived and realized that this is a LIN BUS sensor, Module 08 is setting a 00256. ( AC psi / temp signal no comm)

001 data shows 51 bar / 739 PSI and after seeing box (very cheap looking I decided to scope the sensor, I had good B+ , good ground, and the signal is stuck high at 11.57 volts engine running. Not hint of communication activity. unplug the sensor and the bus goes to 0 when back probing at sensor connector.

Ordered and installed updated OEM sensor (TSB PN 4H0 959126 B) and you guessed it NO Change!

Is there some reset procedure I am missing I did clear the codes , active test compressor comes on however we still have 739 PSI reading and BUS is high with the sensor plugged in.. ( makes me think bad ground at the sensor but it test good (very low mv)

Maybe the heat is getting to me or I should avoid audis 

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Mike Diagnostician
Toronto, Ontario
Mike Default
 

Hi Brian, My initial guess would be the LIN wire between the AC pressure sensor and the Climatronic control module is broken somewhere. Often more than one sensor will share this LIN wire (e.g - humidity or air quality sensors) so if you have access to a wiring diagram it can help you narrow down where to look if this is your only fault code. Because the AC sensor is a LIN slave it will be…

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Brian Owner
Parma, Ohio
Brian Default
 

Right on Mike, Thanks for taking the time to reply and help.

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Mike Diagnostician
Toronto, Ontario
Mike Default
 

Anytime Brian Glad to hear you were able to narrow it down! Mike

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Adam Owner/Technician
Glasgow, United Kingdom
Adam Default
 

Plug in OEM sensor, clear code, get on live data and agitate loom and observe values. Even if it stays high, you’ve got a wiring fault. If you need help checking let me know.

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Caleb Technician
Mishawaka, Indiana
Caleb Default
 

Mike is correct. This diag has nothing to do with Audi. It's just typical LIN bus operation. The A.C. pressure switch is a slave of the Climatronic. It only begins communicating when told to wake up or pinged by the master(Climatronic). You should have a 0-12 v square on the LIN with Pressure Switch unplugged. Because you don't the wire is probably broken. If easy to access why not go the…

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Brian Owner
Parma, Ohio
Brian Default
 

Thanks for the help Caleb! As stated I think I was over thinking the problem. I should have kept a better sense of it being like other LIN bus's.

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Brian Owner
Parma, Ohio
Brian Update
 

Thanks for the input and a much needed Redirect (as I call it) in my thinking! So as I stated I have little Euro experience, and the little that I have been exposed to had some odd manufacturer specific procedure or something. = cheesy excuses lol So we did get the proper part # sensor as stated in original post and signal was same , near 12v flat line. Pulling the control head and checking…

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Matthew Owner/Technician
Atlanta, Georgia
Matthew Default
   

There’s a bulletin for this. I believe the A6 does not suffer from the same wiring issue that the A4 does. I wouldcheck that bulletin.

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Caleb Technician
Mishawaka, Indiana
Caleb Default
 

Cool glad u got it.

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