I have some advice for you that I see hasn't been offered yet. How about trying this:
1.) You'll need to get an inexpensive Bluetooth OBD2 Scanner like the one in the link/pics included. They're about $30 and work great requiring nothing more than your phone or anybtanlet, laptop life , etc. with Bluetooth capability. I've used it with iPhones and Galaxy Tablets, & Multiple Android Phones, a few laptops, Etc. These are very universal if you get the right type: I prefer the VeePeak OBD2 BLE Scanner.
2.) It plugs into your OBD2 Port and connects via Bluetooth. The VeePeak BLE OD2 Scanner does all that most of us will ever need, I've used it in a professional setting too and as my go to OBD2 Diagnostic Scanner, & checked Codes on Countless Vehicles with my iPhone primarily. You can leave these devices plugged in at all times with no issues/problems.
3.) Data from all of the individual sensors is shown in a dashboard format, a graph, or just numerical values.
4.) Use the data logging feature and drive it around to get some readings from the electronics.
5.) You'll need to show this data to someone who relay knows how how to read and decipher it, in person is great but technology makes this easier. You can easily just Email this data to a professional who is a known expert. I can reccomemd one in particular who is the real deal.
6.) Custom Tuners and Progammmers at Performance Shops all around are invaluable with situations Ike this. You've got to get the information from your truck to someone who specialize in GM Trucks which would be easily found, like Nelson Performance who are the Best of the Best or Close in any case, BlackBear Performane, or a similar specialist. There are a few others, but Nelson Performance will send you a data logging unit to use and you can just hook it up & drive around to capture the data, remove the device they provide and send it back to them or in many cases you just email the data to them and things move forward from there. If you have Scanner or data logging capability through any device that's even better!
7.) Call these guys @ Nelson Performance, or a similar business that specializes in GM Vehicles, these guys are Master CERTIFIED with decades of experience. Check it out!
Theyll get serious power gains for you and better hasn't mileage too while keeping everything "safe" and on your improvements that GM can't authorize as they'd be violating by strict epa and other similar laws. Talk to these geaeheads, you'll be pleasantly surprised!
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Im not affiliated with anyone, any public or private's or entity, or anything mentioned- anything I reccomemd or mention is based on personal experience.