I think the on board diagnostics could be very basic, and i concur with ade that its probably a (code reader) tracable fault if it is going into limp (home) mode

you can buy a code reader online that will do 99% of what a garage code reader does, and you can use it again and again without additional cost, it is about 20-25 quid, (eg- an F-super) , it requires a laptop, you can also get stand alone/ handheld ones too- a worthwhine investment for you. IMO

it can also give you other information that may be useful

Previously i mis- interpreted your fault/s (oops)