THOUSANDS of Mercedes-Benz cars costing more than $1 million each could be polluting Hong Kong, but the Government has no way of telling. The Taiwanese Government this week cancelled the quality certificate for six types of Mercedes with 3.2-litre engines after one was tested randomly and found to be in breach of strict exhaust pollution limits. The same limits are in force in Hong Kong, but the Government does not randomly test cars because there is no laboratory in the territory that can do the work. In Hong Kong, as many as one car in nine is a Mercedes, and many of those are in the popular 300, or S, series. Following the Taiwan report, Mercedes-Benz worldwide rushed to control the damage. Mercedes' representative firm in Taiwan was quickly translating the test report into German for the company's headquarters. Two German engineers flew to Taiwan, and the car was taken to another laboratory, which passed the car. Hong Kong distributor Zung Fu was immediately in touch with Taiwan. ''In Taiwan one car in 200 is randomly tested,'' said Zung Fu director of sales and marketing Michael Lee Tat-sing. Zung Fu marketing manager Alex Chan Che-wang said: ''Mercedes cars are tested worldwide, and this is the first and only case where one failed the test, and then only marginally. ''There is absolutely no danger to drivers or passengers.'' Taiwan's Environmental Protection Department tested a Mercedes 300 SEL with the 3.2-litre motor. It failed the test for carbon monoxide (CO) which can kill humans. The limit was 2.1 grams of CO produced every kilometre, but the car gave off 2.2 grams. In Hong Kong, cars powered by the 3.2-litre engine include the 300 SE, 300 SEL and 320 E. Mercedes-Benz are a big seller in Hong Kong. The S-series, which cost about $1.3 million each, is popular because they are much cheaper than other luxury models. More than 500 have been bought from Zung Fu since January this year. The Government tests new and old cars only for visible smoke. It employs 400 ''smoke spotters'', many of them volunteers, to report polluting vehicles. But harmful gases like CO and nitrogen dioxide are invisible, and can only be measured by special equipment.