U codes are a communication problem between computers. They will usually turn on a light in the dash for the area to look at. IE.. ABS,Check engine,Security... Some wont turn a light on, BCM..U codes can come from disconnecting the battery or the battery gets to low on voltage. (like a bad alternator) Last but not least would be a short or broken wire in the communications line. Depending on the year, communication lines were from computer to computer. Newer models splice the commuication lines into one place that feed all the computers.