This 2005 Buick Lacrosse came in with the complaint that the instrument cluster gauges would not work and the T/C , traction control light stayed on. . The customer stated that the air conditioner would have to be turned off and then back on before it would get cold. Through a little bit of research and hands on testing I found that the back up or reverse lights did not work. The trunk release switch did not work and most of the data in the driver’s information center read dashes instead of numbers. Information such as average economy and other fuel data. When I checked codes there were 27 codes stored through various modules. I recorded all of the codes and then cleared them all. There were three that would not clear U1000 in the EBCM, U1300 and P1629 in the ECM. This Lacrosse has the 3.6 liter, LY7 engine.
Since I have already fixed this vehicle I will let you know that researching GM Service Information (SI) will not provide anything but minimal tidbits of information that are useful in diagnosing this problem. Aftermarket information is no better.
In reading service information a code U1000 is for a class 2 data link malfunction. Code U1300 is a generic code for any class 2 module on the car. The description of the code is for low voltage on the class 2 serial data line. Code P1629 is for the theft deterrent fuel enable password signal has not been received from the BCM by the PCM within one second.
Code U1000 should be associated with lack of class 2 communication for one or more modules.
U1300 cannot be retrieved as a current code. If this code is current you are directed to follow the symptom chart for “scan tool does not communicate with a class 2 device”.
Code P1629 should be associated with a no crank/run condition as the theft system will be activated. This car started and ran fine.
When this car showed up at my shop the ECM had been replaced. The BCM had also been replaced but the original had been re installed due to a no crank condition. The work had been done a a GM dealership.
I had installed my tool shown below into the class 2 gang connector under the dash and could see that communication was present in all modules by the flashing LED’s and I also backed that up with a voltage test.
To further confuse the issue SI also stated that the condition had to be present for three seconds. In theory there should have been at least a three second window when class 2 communication was not working with the scan tool. I never experienced that problem.
Below is a picture of the class 2 gang connector located under the driver’s side of the dash.
At this point I am trying to determine if I am going to make the full diagnostic information available for free or if I need to charge for it.
I have a great deal of time and effort invested in this repair and know that with a clear understanding of how these systems work, the use of a good voltmeter, a few common hand tools and my direction, this problem can be accurately diagnosed in five to fifteen minutes. This diagnosis and repair applies to the 2005, 2006 and 2007 year models.
To see the follow up diagnostic information please click here.