Reports that cars could fail the national car test in one lane at an NCT centre and pass in another at the same venue are being investigated, with the possibility of disciplinary action against the tester.
The Minister of State for Transport, Dr Jim McDaid, said he had "grave suspicions" about whether there was anything wrong with the lanes.
"A car which fails a test in one lane should fail in the others," he told Labour's spokeswoman, Ms Roisín Shortall, who had asked about newspaper reports indicating a lack of consistency in the equipment being used in test centres.
Dr McDaid said: "Calibrations are regularly checked, but there can be inconsistencies between lanes.
"I believe the case referred to involved human as opposed to mechanical error."
More than 630,000 first tests and 300,000 retests were completed last year, the Minister said, and he was satisfied that the test results reflected the condition of the vehicles at the time of testing.
Fine Gael's transport spokesman, Mr Denis Naughten, said the perception existed that the car-testing was a moneymaking racket and the objective was to ensure a high failure rate.
But the Minister said Fine Gael had introduced the system in 1996, when in government.
He believed the best method was being used and that a singular entity should undertake the tests, although it was always the case that it would be a money-making system if such a service was put in the hands of one company.
The Minister said he had been "castigated" for supporting the NCTS, the company that operates the system.
There were 57 elements to each test, and a person who failed to comply with them was breaking the law.
"The NCTS is enforcing the legislation in this regard, and the Government is being castigated for catching people who are breaking the law."
There would be a mid-term review next year of NCTS's 10-year contract, and the company would welcome an independent appeals system.
Dr McDaid said that the pass rate for cars registered in 2000 and doing the test this year was up to 70 per cent and getting higher because there were fewer older cars.
