Increased policing of warrant of fitness standards have led to more garages and inspectors being dumped for being "sub-standard".
Thirteen garages and inspector agents, as well as 16 vehicle inspectors, lost their WoF certification in the year to mid-February - double the 2006 and 2007 figures.
The Transport Agency says extra compliance work by its vehicle certification unit is partly responsible for uncovering more warrant of fitness operators and inspectors who are not up to scratch.
The agency says it recently introduced a "mystery shopper" approach, in which a vehicle with a number of faults is taken to an operator. It says this is working well.