Some Fiat vehicles showed irregular emission levels after exceeding the regulatory test-run duration, leading to doubts that the vehicles may have employed a device or software that enables the car engines to throttle back emission management systems, a German probe is reported to have found. The findings of the investigation, conducted by Germany's Federal Motor Transport Authority, prompted many car makers to recall their vehicles to make changes in their diesel engine software technology.

According to the German newspaper Bild am Sonntag, the investigation was held in the wake of the Volkswagen emissions scandal that revealed the car maker had used special software to rig results of emission of harmful nitrogen oxides from its car engines.

During the probe, it was found that an emissions management system on an unspecified Fiat model was throttled back after 22 minutes, while the normal duration of regulatory tests for emissions is about 20 minutes, the newspaper reported.

As an engine protecting mechanism, carmakers are reportedly allowed to throttle back emissions management systems for specified duration – referred to as "thermal window" – to prevent condensation or other damage to engines. The probe reportedly found that while no other company used any software to manipulate emission levels, they however, made generous use of the thermal window leading to variable emission levels.

Auto supplier Bosch reportedly told the German investigators that Fiat too was using a mechanism that enabled its engines to virtually disable exhaust filters. Such mechanisms have drawn criticism from regulators as well as environmental groups.

Meanwhile, top German manufacturers – Porsche, Opel, Audi and Mercedes – said they will recall 630,000 vehicles to amend their respective diesel engine software technologies in response to the probe findings.

A recent study in the UK revealed that around 100 top-selling diesel cars in the country did not meet emission standards and the results of laboratory tests varied with on-road tests.