43 per cent of cars tested an NCT centres in Cavan and Monaghan pass first time.
Figures from the Irish Independent show both counties are below the national average.
1.4 million cars went for a test in one of Ireland's 49 test centres in 2021.
Of the 15,400 tested in Cavan, 41 per cent passed first time, the lowest pass rate in the country, with 51 per cent having a major fault.
1,200 of the first time test failures in the county were for dangerous faults.
In Monaghan, 46 per cent of the 14,100 cars tested last year passed first time.
47 per cent failed on a major issue, and 1,058 being marked a dangerous failure.
Front suspension defects were the most common fault, followed by lights.
Portlaoise had the most first time passes at 60 per cent