An investigation by RTÉ's Investigation Unit has found it is possible for a car to pass the NCT when all of its shock absorbers are defective.

The NCT tests a vehicle's suspension in two ways – a visual check for oil leaks and a mechanical test to identify any imbalance.

The balance test identifies whether there is an imbalance between the vertical movement of wheels on either side of the vehicle.

Provided the wheels are in relative balance – within a 30% differential – they will pass.

However, as oil leaks are not always visible on defective shock absorbers, vehicles with equally defective shock absorbers may pass the test.

This means that cars can pass the NCT even though they may be in what experts describe as "a dangerous condition".

Vehicle experts have told RTÉ’s Investigations Unit that if NCT tests on a car show a reading of above 80 Mahameters (a measurement used to assess shock absorbers) a red flag should be raised because it suggests the shock absorbers need to be replaced.

However cars pass the NCT with readings well in excess of that.

According to test results seen by the Investigations Unit, one car passed with shocks showing readings of 159 Mahameters on one side and 178 on another.

Experts said readings that high would indicate this car was unstable but it passed because the wheels were in relative balance.

In an interview with RTÉ, RSA Chief Executive Moyagh Murdock said the NCT is "highly sophisticated and probably the most sophisticated in the member states involved in the road safety strategy".

She also points out that there is no consensus in Europe on how best to test suspension systems and Ireland is one of only five EU states that do a balance test as well as a visual check on shock absorbers.

The investigation also found considerable inconsistencies between some of the 47 test centres around Ireland, where pass rates can differ dramatically from centre to centre.

Since 2011, there has been a gap of around 19% to 30% between the centres with the highest and the lowest pass rates.

In 2011, 70% of vehicles passed at the Cahersiveen NCT Centre in Co Kerry while the centre in Clifden, Co Galway, had a pass rate of 40%.

In 2012, 67% passed at Cahersiveen but only 39% in Carrick-on-Shannon, Co Leitrim. 

In 2013, 55% passed at Tralee, Co Kerry, but only 36% at Clifden.

58% passed at the Kilkenny NCT centre in 2014 but only 35% at Clifden. 

In the North, where cars are tested at 15 centres, the difference between the pass rates is far smaller, with the figure for both 2014 and 2013 standing at just 7%.