As Martyn has said a PSDATA file will greatly help us with your question, even a screen shot would help us get an idea of what you're seeing.
In terms of understanding CAN we have a large number of resources on this subject. The guided tests are always a good place to start which can show you what to expect.
We have a video on our YouTube channel which goes into some examples of CAN failures along with some more advanced decoding functions available in the software. This can be viewed here -
We then have more information on CAN thresholds and crossing points here - https://www.picoauto.com/support/topic21743.html.
Given it's a Class 8 vehicle, it may well be J1939 protocol of which if you are using the Early Access version of PicoScope 7 Automotive there is a dedicated guided test for this. We have also talked about J1939 extensively in the heavy duty and off highway section on the forum. The following post will help you get started - https://www.picoauto.com/support/viewto ... 85#p101985.
I hope this helps and as mentioned, getting a PSDATA file here will let us take a look and offer some more advice.