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.
Your article “2005 Buick Lacrosse, Gauges Do Not Work” was a tremendous help. The problem started in late 2023 when my 2005 Lacrosse (3.6-liter), which had only about 57k miles on it, suddenly could not be started. When the ignition key was turned, there was only the slight sound of a click somewhere under the hood, that sounded as if it were coming from the driver’s side in the general vicinity of the master brake assembly. However, the battery was strong, because the headlights were bright and did not dim when attempting to start. I then noticed the positive battery clamp was loose enough that it could be removed by hand with a slight twist. I tightened both clamps, but the car would still not start. It could also not be jumped. I had the car transported (AAA, no charge) to a small-operation mechanic who later told me the battery, starter-relay, and starter were fine but the exact diagnosis was beyond their capability, so I paid them $125 and we started the car (by removing the starter relay and jumpering the two starter pins, then reinstall the starter relay) and drove it to a larger mechanic’s shop (not a dealer). After explaining what had taken place, the 2nd mechanic’s shop replaced the ECM with a refurbished one from eBay (approximately $200 for the part, only). I paid $507 to this mechanic when I retrieved the car, but soon noticed that none of the dashboard instruments were working, the dashboard information (for example how many miles can be driven before running out of gas, etc.) was replaced with dashes, the reverse lights were not working (it remains undetermined if there was a trunk release issue). I did some online research and found this series of 3 articles on Sparky’s Answers, and saw that the symptoms described were identical to mine. I notified the mechanic of this, and of the articles I had read on Sparky’s Answers (he was skeptical, at first) and he replaced the ECM again, but this time with an AC-Delco (approximately $600 for part cost only). This 2nd ECM replacement completely corrected all problems described herein, and I paid another $650 to this mechanic. My total cost was just under $1,300 (for both shops, including all parts, labor, programming, and $45 for “Flash Download”), which is a helluva a lot for this old (albeit, low-mileage) car – but I suspect it could have been a lot more had I not come across your well-written and useful articles. I am an electrical engineer, and worked on cars when I was younger (in the 1970s & 1980s) and almost regret that I did not fix the car myself, but i suspect the programming and need for a flash download would have tripped me up and wasted a lot of time. In closing, I wish all technical articles, reports, etc., I read were as informative and as useful as yours were in this particular matter.
Thanks for the kudos and I am happy that my articles helped you get your Buick back on the road. Something that I try to get my customers to realize is that the $1300 you spent on this repair would only be two months payment on a new vehicle and you would have another 58 to go.
Sparky – another question regarding a 2005 Lacrosse, please. After changing the ECM to fix the instruments issue addresses above, I hired a locksmith through AAA to create a new FOB-key. Unfortunately, I had no working FOBs for him to use to program a new FOB. After spending an hour trying to get the car to “enter programming mode for the new FOB” using his electronic tools, he gave up, saying there must be something wrong with the car’s “keyless entry module”. I informed the locksmith that the ECM had recently been replaced, and he said ‘maybe’ the mechanic “did not sync the new ECM with the keyless entry module”. What do you think might be the problem, and potential solution?
The ECM is not connected to the programming of remotes. A scan tool must be used to enter the programming mode of the remote control door lock module. Once it is in programming mode a good remote must have the lock and unlock buttons pressed at the same time. When the module sees those two signals it erases any old remotes from it’s memory and learns the new remote. So there is either a problem with the new remote, the RCDLM or the scan tool interface. The first suggestion would be to test the new remote to make sure both the lock and unlock buttons work.
So what fixed it
The two follow up articles explain that the engine control computer was the cause and it likely died from poor battery cable connections.
So you are saying it needs a new pcm and if so dose it have to go to the dealer to be reprogrammed
If you have the same test results that I did, then yes the engine control computer will need to be replaced. Yes the computer will need to be programmed and relearns performed. Also the battery cables need to be closely inspected and repaired if needed since a battery event is usually what causes the computer to fail.
So would it need the ecm or the pcm?
The ECM as shown in the second article. I have to state again that you need to make sure your vehicle, symptoms and test results are the same as in the articles. Also the repair cannot be completed without programming and you will need the assistance of a technician with the correct scan tool and access to GM software. Furthermore any underlying battery issues have to be corrected or the replacement ECM will fail.
Also head lights keep goin out
The cluster, reverse light and a/c problems are likely due to a faulty PCM as outlined in the articles. The headlight issue could be many different things but are probably not related to the other problems. You would first need to have the codes read and compare all symptoms in the article to yours.
It’s all the things in your article same problems
If it is exactly as in the article the ECM is bad and will need to be replaced. Replacement requires programming and set up. It is critical to correct any battery cable issues before replacing the ECM. Make sure you read all three articles about this vehicle.
My cluster and reverse lights does not work also ac has to be turned of and on please help