What does U0002 mean?

U0002 is a performance or quality complaint about the high-speed CAN bus rather than a total failure. The network is still passing messages, but with enough errors, corrupted frames, or brief dropouts that a module decided to record it. That is why the symptoms are so frustrating: warning lights that come and go, systems that drop out and return a moment later, codes that clear themselves, and electrical gremlins that will not reproduce on demand in a shop bay. Intermittent faults have intermittent causes, and they are almost always mechanical connections rather than electronics. A CAN wire that has chafed down to a thin strand, a pin that has spread slightly and no longer grips, a connector with green corrosion or moisture inside it, or a ground strap that is tight but corroded under the ring terminal will all pass a static test and then fail when the car flexes, warms up, or hits a bump. Weak battery voltage is the other classic cause, because modules that brown out briefly during cranking or high electrical load look exactly like a marginal bus. The way to catch this is to test while the fault is happening. Start with the battery and charging system, then scan stored and pending codes across every module so you can see which ones are dropping out. Watch live data while you wiggle-test and back-probe the CAN wiring and connectors, flex the harness at bulkheads and along brackets, and load the electrical system. Inspect grounds and any connector that has been opened during past work. The temptation with U0002 is to replace a module to make the ghost go away, and that is exactly the wrong move — intermittents are overwhelmingly wiring and connector problems, and a programmed module is an expensive way to learn that. Save module replacement, and the terminating resistor, for after the connections have been proven good under stress.