You have described a faulty selector switch (known as a commutator at Smeg)
As you have changed the selector switch and it hasn't worked at all, I'd suggest checking the part number on the new switch against the part number on the old switch.
If it's different then it will need to be rewired to suit the new switch.
Smeg can provide this information if you are an engineer and if you purchased the new switch from them. Otherwise, they simply refer you to your supplier.
Check the clock, if set, check the part number on the old switch against the new. (should be printed on the switch).
Let me now what you find, I do have a small selection of technical bulletins on Smeg commutators and once I know the number of the old and new switch, I can check.
Hope this helps
If your oven has a clock, did you set the clock to take it off of Auto operation?
Thank you for your help. I spoke to the suppler of the switch who stated that there is only one switch for this model of oven. I've asked about the possibility of the replacement switch being faulty, and I'll let you know what they say/whether a replacement commutator block resolves the problem :-).
Yes, there's only 1 switch listed, however, it may be a superseded switch.
Check the number on the original switch against the new one.
It turns out that the clock wiring was dodgy. The new switch works and the clock is fixed, so hey presto, I will be having roast chicken tonight :-)
Thanks for your help.
Thanks for letting me know.
Good luck with the appliance.