Car Clinic: your motoring problems solved

The key to my locking wheel nut is missing; now I have a flat tyre and cannot change it. Any ideas?
KT, Croydon

Locking nuts are sold with a code, so owners can order a new key from the maker. If you don’t have the code, garages have tools able to remove most nuts. Breakdown organisations such as the AA and RAC can also usually help. A last-ditch DIY solution — provided you have a simple type of nut, without a spinning outer shroud/collar — is to take an old 12-point socket that is a little too small, hammer it until it bites onto the nut, and then twist it off with a ratchet. Don’t try this with a high-spec locking nut from the likes of McGard, though: you’ll destroy the wheel.
Dave Pollard

Can fuel additives or retrofitted devices make a diesel car less polluting? And will removing biofuel reduce NOx emissions?
MH, Wetherby, West Yorkshire

It is possible to fit older vehicles with kit that will reduce gases to meet emissions standards, but it is financially viable only for commercial vehicles. Most diesel contains an additive (premium fuels often have stronger ones, or you can add your own) that clean the engine, helping it to run more efficiently and thereby cutting emissions. UK diesel contains up to 7% biofuel, and under heavy load it can create more oxides of nitrogen (NOx), but the effect is negligible compared with that of poor maintenance or a harsh driving style.
Tim Shallcross