FRANKFURT (Reuters) - A German emissions probe conducted following the Volkswagen VOWG_p.DE scandal suggested that some Fiat vehicles showed irregular levels of diesel exhaust pollution if the cars ran for longer than 22 minutes, Bild am Sonntag said.

A logo of Fiat is pictured on the Fiat Panda at the 86th International Motor Show in Geneva, Switzerland, March 1, 2016. REUTERS/Denis Balibouse

A spokesman for Fiat in Italy declined to comment.

The newspaper said the investigation by the Federal Motor Transport Authority (KBA) revealed that an emissions treatment system on an unspecified Fiat model was throttled back after 22 minutes, a sign that the producer may be employing technical devices or software to avoid emissions cleaning filters.

The normal duration of regulatory tests for vehicle emissions is about 20 minutes, the paper added.

The Volkswagen emissions-test cheating scandal cast a spotlight on industry-wide emissions of health-threatening nitrogen oxide (NOx) levels in diesel fumes.

Following the KBA investigation, the firm and some other top German manufacturers -- Porsche, Opel, Audi and Mercedes -- agreed to recall 630,000 vehicles to tweak diesel engine software technology.

While no German manufacturer other than VW was found to have used illegal cheating software, carmakers were making liberal use of something they call a “thermal window” which refers to the time when carmakers are allowed to throttle back emissions management systems in order to protect the engine from condensation or other damage.

Auto supplier Bosch [ROBG.UL] is said to have informed German investigators that Fiat was using a mechanism that virtually disabled exhaust filters, Bild am Sonntag reported. Bosch declined to comment.

Bild am Sonntag did not clarify whether the mechanism revealed in the Fiat test also functioned as a “thermal window”.

Regulators and environmental groups have criticized the widespread use of systems that switch off emissions treatment not just to improve engine performance but also to increase the interval between services.

The government official said there needed to be clarification at European level over the use and declaration of engine protection mechanisms.

(Story refiles to clarify garble in fourth paragraph).