A UK-wide crackdown on the illegal use of diesel in cars had nailed just ten offenders in the past week - but two of them were caught red-handed in Northern Ireland.
Those motorists are believed to have had their cars impounded - and have to cough up more than £250 to get them back, including duty on the fuel they've used.
Customs and Revenue inspectors targeted motorists who pulled up to red diesel pumps in petrols stations.
Red diesel is taxed at a much lower rate and is meant for use by tractors and some other off-road business vehicles.
The motorists had their tanks dipped by inspectors to see if they were already travelling illegally on the low tax red or green fuels. Green is the Republic's version of red and is also illegal on Northern Ireland roads.
Drivers who had legal white diesel in their tanks were issued with leaflets warning them not to fill up on red.
But the ten people caught with red in their tanks - including the two from Northern Ireland - are believed to have their cars impounded.
This represents a drop of three quarters from the last red diesel operation.
More motorists may have escaped this crackdown, known as Operation Hummock, because it was well publicised before it started.
But Customs officials say they're satisfied with the limited number they caught, because their main objective was to warn motorists to stay away from the illegal fuel.
Colin McAllister, the national oils strategy manager, for Revenue and Customs, said he was " delighted with this week's results".