Infrastructure minister Nichola Mallon has commissioned reviews to address how issues with the MoT in Northern Ireland escalated to the point that most tests had to be cancelled with immediate effect.
Speaking in the Assembly, Ms Mallon said she has instructed the Driver and Vehicle Agency (DVA) to reopen test centres as soon as possible.
But she said she's "concerned" by the short time frame between when cracks in lifts were first noticed in November to when lifts were removed from action towards the end of January.
She's now commissioned reviews by auditors and engineers to establish what information was available around the time the cracks were discovered and how inspections were carried out into the lifts themselves.
"That's why part of the measures I've put in place to address the situation include two reviews, one review is independent of my department by auditors because I want to understand what happened here and find out who knew what and when and what action was taken," she said on Monday.
"A key part of that will be the contract that was involved in the inspections. The second is via the appointment of independent engineers. I've asked them to provide me with independent expert advice about the steps I need to take."
The minister said two new lifts have now been opened and three other lifts cleared for use to address the demand for tests, but she did not specify an exact date when 15 centres would become fully operational again.
She said safety of staff and customers was paramount but that the current situation is "not acceptable".
Ms Mallon said she's frustrated it's not possible for her to issue four-year-old vehicles with an exemption as she has done for other vehicles.
"Testing on heavy goods vehicles and buses is continuing, heavy duty lanes are also being used and MoT opening hours are being extended to prioritise taxis and four-year-old [vehicle] customers," she said.
But she said politicians in Northern Ireland do not have the power to omit the underbody inspection on the vehicle test as is currently ongoing in the Republic of Ireland, where similar problems are being experienced.
Speaking to the Department for Infrastructure committee last Wednesday, Ms Mallon said she was seeking legal advice on moving the MoT from every year to every two years.
The chief executive of the DVA Paul Duffy revealed to the committee the use of lifts by the Irish National Car Testing service (NCT) had also been suspended after they noticed cracks in lifts.
Mr Duffy said the lifts at fault were installed over two years between August 2011 and November 2013.
The lifts, which Mr Duffy said were regularly inspected, could cost between £30,000 and £40,000 each to replace.