worth resetting the ecu imo before u take it back and if it pops up again then change sparks over and see how u get on
on my girlfriends corsa i changed the HT leads and it came up showing knock sensor system ECU failure, cleared faults down and put old leads back on and it was fine, put any other leads on and the same happens again
strange i know
even though u are covered on warranty it is still a pain getting it sorted and vxhall will go over the car with a tooth pick to get out replacing the part for free
are they aftermarket spark plugs ?
the problem doing the pedal trick is that it will show up stored codes as well so u wont know what is active and what isnt