Replaced 12v battery now Master Warning light on

Some will advise just disconnecting the 12 volt battery again for a few minutes to see if that clears the condition and it doesn't come back.

Not much to lose doing that, as you already lost most settings presumably replacing the battery anyway.

If the MWL stays on, there will be trouble codes to tell you why. Just read those, they will often indicate something simple, like some parameter needing to be relearned, with usually a simple procedure for doing that. Or it might be something else. Won't know without getting the codes.

 

Did you fully charge the new battery before installing it ?

 

Just because you read that somewhere does not make it true.

It is NOT true. Or at least you should not take the chance.
"Mail order" batteries usually come near to DEAD, for instance.

You should charge it up good NOW because it might be limping along with a partial charge and that is BAD for battery health and longevity.

And now would be a good time to check the charging voltage when the car is ON (ready) too.

 

Maybe yes, maybe no? It wouldn't hurt to hook up a smart charger, let it run a full session. Check Owner's Manual, but I believe gen 2 OM stipulates 3.5 amp max. And that's probably a bit debatable; around 4 amp is not pushing it.

If you want to SEE where your new battery is at, an electronic load tester will do the job. Automotive retaliers selling batteries typically have them now, and will test for free, hoping to get and/or keep your business. Dealerships also have the testers, but more-n likely will charge a "diagnostic fee". They don't just check volts, like a multimeter: they also determine the as-tested Cold Cranking Amps, and give a diagnosis.

Pick up something like Solar BA9, an entry-level tester, and you can test for yourself. Good investment.

 

A 4 amp capacity charger almost NEVER really puts out 4 amps.
If it does, there might be a problem that only a new battery will solve.

And you don't need a load tester to get a very good idea of the health of your battery.
A charger, voltmeter and appropriate chart is all you need.