Fairly good (if that was measured after many hrs non-use).

 

My mechanic tested my battery and it said weak battery, and advanced auto said good battery, 12.55. Both tests were done 5 mins after shut off.

 

12.55 sounds about normal for a sitting battery, on the lower end but not completely gone. The correct test would be a load test, not just measuring voltage with the multimeter, to see if the battery is still holding its full charge capacity. That should be what both your mechanic and advance did..

 

If starter crank happily - good battery, if it struggles - bad battery :-)

 

Thank you. Car starts like a dream.

 

On cars like these with many modules, sorry but no longer true.

They need the volts to stay good during cranking and also at all other times or expect grief (RP, spurious fault messages, codes, misfires, etc).