Defects in emissions testing: Modern diesel cars fail more often

The defect rate ranged between 1.8 and 9.3 percent, depending on the vehicle category. This is according to the current final report of the Federal Association of Motor Vehicle Trades (BIV) on the AU defect statistics.

The report's database includes a total of 10.8 million vehicles examined from approximately 27,400 companies that perform exhaust emission inspections, which were statistically evaluated using the industry software "AÜK Plus." This includes passenger cars, commercial vehicles, and motorcycles subject to exhaust emission inspections.

In total, specific defects were identified in over 400,000 vehicles during the vehicle inspection. This corresponds to an average defect rate of 3.8 percent – ​​a slight decrease compared to the previous year (2023: 4.1 percent). The distribution of defects across the different vehicle types has hardly changed year-on-year.

Detlef Peter Grün, Federal Master Craftsman and Vice President of the Central Association of the German Motor Vehicle Trade (ZDK), emphasized the importance of regular emissions testing: "The current figures once again underscore the high relevance of emissions testing for environmental protection. Only a tailpipe test can ensure that a vehicle complies with legal emission limits throughout its entire service life."

Since July 1, 2023, diesel passenger cars and commercial vehicles meeting the Euro 6/VI standard have been subject to a new measurement procedure as part of the exhaust emission test: particle number measurement (PN measurement). This was introduced to better detect emissions violations – with noticeable effects. Significant deficiencies were also identified here: The defect rate for diesel passenger cars was 4.0 percent – ​​despite passing the on-board diagnostics (OBD) test.

Grün emphasizes: "This rate clearly shows that OBD testing alone is far from sufficient to detect defective diesel vehicles. The investments by the emission control agencies in modern measuring technology – now over 125 million euros – were necessary and effective."

A joint interim assessment of the new testing methodology was presented for the first time in February 2024. The evaluation is the result of a collaboration between the TÜV Association, the German Central Association of the Motor Vehicle Industry (ZDK), the ASA Association, DEKRA, and the KÜS (German Institute for Safety and Health). It was based on the results of approximately 950,000 diesel cars tested between July 1 and October 30, 2023.

Approximately 32,000 vehicles failed, corresponding to a failure rate of 3.43 percent . A clear correlation with mileage is evident:

• Vehicles with less than 50,000 kilometers mileage: 2.8 percent failed
• Vehicles with 50,000 to 160,000 kilometers: 3.3 percent
• Vehicles with over 160,000 kilometers: 4.7 percent

What's striking is that only 0.72 percent of the vehicles tested showed a fault in the exhaust system that was detected via the on-board diagnostics (OBD) system or stored in the control unit. This underscores the claim of the trade associations that purely electronic diagnostics alone are not sufficient to reliably detect emissions violations.

The report highlights the positive finding that emissions-related defects were identified and professionally remedied in approximately 185,000 vehicles prior to the actual vehicle inspection. These vehicles subsequently passed the inspection with permissible emissions and diagnostic data.

According to the German Automobile Industry Association (ZDK), the annual number of emissions tests conducted is approximately 25.5 million vehicles. This includes both vehicles subject to emissions testing as part of the general inspection (HU) and additional tests, such as those required during workshop visits or in suspected cases.