Authorities and car manufacturers have been on alert in the wake of the Volkswagen emissions scandal, which emerged in late September.
Volkswagen (VW) admitted its diesel-engine cars had been fitted with computer software designed to flatter emissions data during tests.
In the days following the scandal several car manufacturers issued statements affirming their cars adhered to all emissions standards and were not fitted with cheat software.
VW said around 500,000 cars in the US had been fitted with the devices, which were designed to make VW cars appear more fuel efficient and environmentally friendly than they really were.