Why is it that sometimes when the key is put into the ignition the dash lights come on momentarily then go out and nothing happens? Have changed the battery in the fob.
The problem could be with the igntion module or as known in Mercedes world EIS. Your key battery is not the issue here. You need to scan the codes. It could also be a loose ground.
Does the car still start ok every time?
Only when the dash lights are on. When it is dead, it is dead. Have to remove key and try again. So far it has not left me stranded but it can take 5 or 6 goes reinserting the key to get the dash lights to stay on!
Have you had the codes read? Not check engine light. Do a full system scan. Ideally a mechanic that has Star Diagnostics or the dealer. It would be worth it to spend $100. Make sure to get a print out of the codes.