I have a 2004 Chevy Tahoe with about 180k miles. Its really taken care of with regular maintenance. Recently after driving 30min on the highway, the next time I turn it on white smoke comes out the exhaust. I also noticed it is burning my oil and coolant, having to add more almost every month. It just had an oil change and inside the filter the oil had turned to jelly. I've asked a few mechanics but they all say they wont know til the check engine light turns on. Really trying to find a solution since I love this car
The motor has a failed head gasket, and/or a cracked cylinder head, and/or a faulty oil cooler. Any of these would explain the loss of oil and coolant, and the jellied deposits (from coolant getting mixed into the oil) inside the oil filter. Here's an almost-identical question and answer.
The mechanics you've asked are lazy. This is not an OBD/software challenge, but is a hardware diagnosis challenge...and not a difficult one, IMO.