Hey guys, I'm sure there are many threads addressing this but I thought I would add my experience in resolving this issue in case someone was doing a quick panicky search looking for answers. Sometimes the solution will be buried pages deep in a thread a decade old. I have a 2011 Mustang GT and last weekend I decided to replace the spark plugs. Most of you know where this is going but for those that are looking to do the same please be aware of the following:
Disconnect the battery before replacing the spark plug in cylinder 4 due to the risk of shorting out the circuit breakers (that's what Ford calls them) mounted on the side of the positive post with your socket wrench. If you rationalize that you'll just be super careful...rethink that position. While the circuit breaker assy is relatively cheap, it can be difficult to find in most part stores leaving your baby down until you get one. I wound up shorting out the 100A fuse that powers the power steering control module thus generating the AdvanceTrac Service message and nearly un-driveable vehicle.
Granted, this isn't the only way to get that condition but it is one of the easiest and cheapest to fix so be sure to check those fuses first if you get that message.