hi
if thats 11.9 with engine off, yes thats too low, that would drop to about 10v I bet once the engine cranking
put it on charge overnight and see how it goes, if it drops again, will be new battery
if your cars a diesel, that will causes you lots of faults once the pre heater kick in on start up
did you check it on start up, should be over 14.5 volts
cheers
Joe