P0170 fuel trim malfunction Mazda?

That's really good information, especially the spark plug color. Fuel trim codes are notifications that suddenly something has changed in the way the engine uses fuel, even though it can compensate. The spark plug is a good indication there is a vacuum leak on the #2 branch of the intake manifold or other reason that cylinder is running lean. Look for vacuum hose fittings on that branch and see what's up with them. If that gets you nowhere, suspect the injector or possibly the intake manifold. Fuel injector cleaner is a cheap and easy thing to put in the mix at this point, too.