2005 Nissan Primera

Engine fault appears saying PO171 Fuel System Lean fault and fuel economy has skyrocketed. 2 mechanics have said can be many things and don't want to really get involved. Anyone else had similar issues?

Also key fob not working even after changing battery. Has been playing up for a while where I need to make a clicking noise when key is taken out of ignition but now is not working.

Thanks in advance

I would start with any serviceable items that are overdue, oil, filter, air filter, fuel filter, etc then I would also be checking for the condition of rubber vacuum and breather hoses to see if they are split or perished. This can usually be identified by a hissing noise with the engine running at idle.
Replace parts if you find any damage to the hoses or gaskets around the intake system (inlet manifold, etc)
Alternatively, it may be something as simple as an exhaust leak - place a rag over the rear silencer tailpipe with the engine at idle speed (outside in the fresh air to avoid carbon monoxide poisoning) and if you hear a hissing from the exhaust front section, manifold or oxygen sensor(s) then that is your fault.