Since Thursday 16, those trying to book a test through the online system have been unable to do so.
Customers with an existing booking are unaffected by the issue.
The agency said it was trying to resolve some technical difficulties but the service would be operational by 10 am today, Wednesday, September 22.
The agency said the system upgrade began at 5 pm on Thursday, Sept 16, and had been unavailable since then.
“The DVA is upgrading and improving its MoT and driving test booking system,” the agency said.
“The go-live date for the new system was delayed in order to resolve some technical difficulties associated with the transition.
“The DVA apologises to customers for any inconvenience caused.
MoT test checks are carried out to ensure a vehicle meets road safety and environmental standards.
It is against the law to drive a vehicle of MoT age without a valid MoT certificate and those caught doing so could be fined up to £1,000.
The DVA says it has consulted with the PSNI and the Association of British Insurers to make them aware of the situation.
“The PSNI has agreed not to penalise the driver of a vehicle whose MoT has expired so long as the vehicle is in a roadworthy and safe condition; the vehicle is properly insured; and the driver can provide proof that a test appointment has been booked for the vehicle,” the agency said.
“The ABI view is that not having a valid MoT certificate would not necessarily invalidate your insurance, but if you are in that situation due to delays to vehicle testing in Northern Ireland then it is important to check your policy documents or speak to your insurer.”