Vauxhall Zafira lights

came on the other day about my bi xenon lights not staying on full beam no longer than 2 seconds any way took it back to vx garage had strong words with them to cut a long story short will not be able to go there again phoned vx customer care and ask for help with the cost of parts was told more or less no chance they told me to take it to another vx garage 42 miles away decided to let a good local garage who put it on a dinostic machine and it came up with po621- code which is generator lamp "L" control circuit mulfunction,i know this is the alternator but mine seams to be working no lights on the dash checked battery and fuses all working fine car starts fine could there still be some think wrong with my alternator yours mitchy.