Sounds like you have a charging system problem. Unfortunately the insane engineers have seen fit to hang unreliable, sensitive computers onto every part of our cars, so all those seemingly unrelated systems are affected by low system voltage. The radio code is just a trick designed in to force you to go back to the dealer and spend more money to use what you already own. There is a really good chance everything else will work once the electrical system is working properly.
Start by charging the battery at a slow rate for an hour. Next, turn the charger off and remove it, then measure the battery voltage. If it is good and fully-charged, it will read close to 12.6 volts. If it is around 12.2 volts it is good but discharged. If you find it is around 11 volts, it has a bad cell and must be replaced.
Next, measure the battery voltage again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator. If it is okay, that proves it is okay to have the second half of the tests done by your mechanic. That involves measuring "full-load output current" and "ripple voltage". If one of the six diodes in the generator is defective, that will reduce the maximum output current to exactly one third of the rated current and ripple voltage will be high. 30 amps from the common 90 amp generator is not sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
Be aware that the most common generator failures, especially at the mileage you listed, are caused by worn brushes, and that almost always starts out as an intermittent problem. The generator will stop working intermittently for longer and longer periods of time until it finally stops working completely. Any testing has to be done while the problem is occurring. That means if testing shows it is not working, it must be replaced. If testing shows it is working, that only applies to right now. You have to catch it while the problem is acting up. That can be found yourself with the voltmeter by measuring battery voltage while the engine is running.
Friday, February 26th, 2016 AT 7:34 AM