Saw this first last year in a 19 Impala, thought it was a fluke and a one off. IP would cut out with several warning lights, many U codes. Could see on scope both high speed CAN and low speed GM LAN going whacky. It was intermittent and made no sense whatsoever. Decided to replace the BCM since the only place busses come together. Did not help, I was stumped. Finally discovered thet the LED conversion headlight bulbs, more specifically the little fans were causing the problem. Auto headlights were on inside shop. Turn the lights off, perfect data signals, turn them on and goes crazy again. I still don't know how it can backfeed the BCM driver circuits. On the Impala either light plugged in alone would cause the problem. Also the fans on the "bulbs" were noisy. I scoped the driver circuits and couldn't see anything, so I'm assuming must be RFI from the little fan motors somehow interfering with BCM. These bulbs were installed 6 months prior with no trouble until there was.

So not a one off, now had a 2015 Silverado with same bus troubles. IC would cut out, with many U codes in different systems. U0078, U0140 and U0155 primarily. Then I noticed the High Speed CAN bus was upset too, and thinking of the BCM before eventually remembering have seen this before. Interesting to note the CAN bus really is error tolerant, garbage for signals yet PCM and other CAN devices could "see through" the crazyness, not so much with GMLAN. I captured the low speed bus on scope could see it chage from clean to dirty with headlights on. I would attach here if I could, I will share them anyone interested.

So moral of the story: LEAVE IT ALONE! Trying to make something better than the design is always a bad idea.

Kevin Baker
Technician/Manager
Autocraft, Inc
Glenburn, Maine, USA