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Some owners claim they were duped into believing the repairs were mandatory.

Volkswagen's optional repair program for millions of emissions-cheating TDI vehicles has amassed numerous complaints of alleged driveability problems and other issues.Common gripes include loss of power, increased fuel consumption, shifting irregularities, EGR valve failures and stalling, according to a long list of complaints on the popular UK automotive site HonestJohn and customer testimonials directed to The Guardian.

With nearly a half million VW Group TDI vehicles already serviced in Europe, some of the complaints are undoubtedly coincidental but frustrated owners argue that thousands of similar observations are evidence of a trend.

One Audi A5 TDI owner commissioned a dynamometer test before and after the repairs to determine if the 'fix' would affect power output. The chart shows a drop of up to 65 pound-feet of torque at 2,000 rpm, with 20 fewer horsepower, though the output curves merge around 2,600 rpm.

"The results are quite damning and back up a lot of what has been said here about ~1500-2500 rpm and loss of power and torque," said BigAndyG. "Audi ... wouldn't engage on showing me their test data but just trotted out the old 'maximum torque is not affected' line - which is true but very misleading for driveability etc."

To be clear, EU complaints relate to repairs that are unlikely to be approved for US-market vehicles due to different emissions regulations. Newer 1.2- and 2.0-liter engines have been brought into compliance with a software update, while 1.6-liter mills also require a new airflow 'conditioner' for improved MAF sensor accuracy.

Some of the owners experiencing trouble with the repairs were doubly irritated to learn that the program is voluntary. A few owners who brought their vehicles in for unrelated service claim the repairs were applied without asking and despite an explicit request to opt out.

Before launching the repair program, VW already faced customer backlash in the EU after declining to extend its US compensation and buyback program to its home continent.