Cleaner diesels, so long as the sun is shining

In road tests following the VW scandal, diesel cars were shown to produce more exhaust pollution in colder weather

Diesel car tests by the UK, French and German governments in the wake of the VW scandal answered some questions but raised many more.

In official tests, cars are gently accelerated to 30mph (50kph) and then slowed down several times on rollers in a laboratory. This is then repeated to faster speeds. All diesel cars passed. The UK testers then reversed the order, starting with the faster part.