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Hi, I apologize in advance for how long this post is.


I bought this truck a little over a year ago and for the last 10 months I have been battling this intermittent problem with I think some wiring, but I can’t figure it out. Given that I’ve already taken it to the mechanic twice and they also have no idea, I’m just trying to see if anyone has any ideas or dealt with something similar.
The first time, January, it acted up I was on the highway. Going about 70 when the speedometer suddenly started going down to about 40 then jumping back up to 70. It had only done it the once so I didn’t think much of it other than a speedometer that was starting to go bad. About a month later my speedometer seemed to be working okay yet, only had done it once or twice since the first episode. One day though, the battery light and check engine lights came on and my tire pressure was flashing with the message low power mode. I turned it off and back on; everything had gone away except my check engine light which went away later that day. Fast forward a couple months I had had any problems. Then my RPM dropped to 0. This was the last time the problems were that spaced out. After this incident my speedometer and rpm have intermittently not worked. Some times at the same time. Sometimes just one or the other. There was another incident shortly after my RPM stopped working where I got another message and a check engine light and again the flashing tire pressure light came one this time it said Serv tmp. Later that month the battery died for no reason so we replaced it figuring it hadn’t been replaced recently and it didn’t have a reason to have died (the lights were not left on or anything it just refused to roll over without a jump. It tried really hard but it was just a tad too dead). In April it started lurching. I can’t induce it, it just shifts very hard when you hit 15 mph and then again around 25-35 and sometimes 45 mph as well. This lurching is completely intermittent and every time I try to show someone else it won’t do it. Flash forward to about June my check engine light comes on. Check my oil which is low so I bring it in to the auto shop and have them run my codes to be sure it’s just the oil because the oil change light hadn’t come on. At this point, I finally cracked and brought it to the mechanic because there were a lot of codes. The mechanic told me to just clear the codes and run it for a while to see what comes back. My dad borrowed a fancier scanner than the auto shop to see what the codes were before we cleared them. Then I drove it around a few months nothing too out of the ordinary for this truck until one day the light on the dash decided it didn’t want to line up with which gear I was in. I drove about a mile (forward) in what it thought was reverse. The lurches during the shift at this point were VERY bad so I made an appointment with my mechanic again. He was pretty booked out so my dad tinkered on it for those few weeks. He once again read the codes and there were some new ones as well as old ones that had popped up. He cleaned the grounds up and checked again to make sure the codes were still there before I took it to the mechanic. They were. I bring it in to the mechanic. There are absolutely no codes. Not even the history codes. He can’t get it to be symptomatic either (although it still was after I got it back from my dad). The mechanic said I needed some transmission fluid, so we added that. Drove it some more and my check engine light comes back on and shifts poorly for a week as well as not registering that I’ve switched from park/reverse/drive. For the whole week, I had a problem every day. I drop it off at the mechanic and he can’t find a single thing wrong with it and cannot get it to be symptomatic. He test drove it three times on two different days including after it rained (I’ve noticed that it does it more often after it rains or the dew has set or after it’s been sitting for a few hours. Like it will work the first time that day but not the second time). I get it back from him and it drives well for about a week and a half then it starts intermittently shifting weird again. Today the odometer has also stopped working. The codes I’ve pulled off of it so far are the following: P069E, P0700 (recurrent), U0073 (recurrent), U1301 (recurrent), U0100, U0140, U1305, and U0773.

Does anyone have any ideas? I’d really like to get this fixed before it stops working completely. (Sorry again for the long post!!)
 

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Welcome to the Nation.

It would help if we knew the Make, Model, Engine of the vehicle . Due to the DTC P069E and P0700 I'm assuming it is a model year 2008 or later. Are there any aftermarket products installed in the truck and connected to any electrical wire?

I think you are going to need to find a technician with an oscilloscope to help you fix this problem. Someone with a high end scan tool may have a built in oscilloscope.

The Modules (ECM, BCM, TCM, etc) in the truck, communicate using the Class2 Serial Data Bus or the High Speed GMLAN Serial Data Bus or both. All of the 'U' codes are related to problems with one or both of the Data Buses. Almost every function in the Cluster is controlled by Modules (usually the ECM or BCM) using one of the Buses.

Something is interfering with the operation of the Buses. Could be one of the Modules or a short in the wiring. A tech with an oscilloscope can connect to the Buses at the OBD connector and read the signals on the circuits. The fact that a scan tool can read the codes indicates that part of the Bus is operating.

Without equipment, about all you can do is try disconnecting Modules, one at a time and see if it changes anything. If the truck is not buried in snow, maybe try disconnecting the Fuel Pump Control Module first. It is located under the bed above the front of the spare tire. That Module is the most likely to sustain damage or corrosion. You can erase the codes with the Module disconnected and see what comes back up. Obviously the truck won't run with that Module disconnected

P069E indicates the Fuel Pump Control Module is telling the ECM to turn on the 'check engine' light. P0700 means that the Transmission Module is telling the ECM the same thing. It doesn't necessarily mean the Modules are bad, but they are not happy.
 
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