Coming late to this as it’s in the caravan maintenance section that I wouldn’t normally look at, rather than the Motorhome section. I have no specific advice except to suggest you join the free Autosleeper Forum (the Auto sleepers Owners Club ASOC costs a wee bit). If you can’t find the question by searching, like you would here, they’re very helpful (like UKCS folks). And the regular newsletter is interesting.
4,5 and 6 flashes are all various control board failures, in your case 6 is a combustion board failure which will need repair.
As far as removing is concerned, first and hardest problem on AS motorhomes is getting at gas connection, it's up against the wall and often accessed with a 17mm spanner through a cutout at the back of the cupboard to the side, but not always!. Second, release inlet and exhaust pipe from body work outside plus cutting any sealant from exhausts delicate heat shield so freeing it can later pass freely through the floor along with exhaust. Finally the remove blown air duct pipes from heater, as well as the heaters two top mounting brackets.
Then hopefully with a helping guiding hand outside, it is a matter of careful gentle persuasion to pull it off it's lower mountings and remove heater still complete with outsides pipework intact.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets