The car had been sold at auction to a friend of victim Rafel Jeanne, who was driving on the night of the fatal crash. The car had previously been involved in a crash in Scotland.
It failed an MOT carried out on 9 February 2023 with the warning system light on, indicating a fault with the seatbelt, air bag, or both, and with the mileage recorded at about 98,000.
Lewis retested the car two weeks later, telling police in interview when he did so "the seatbelt and warning light were now off and the rear seatbelt was now working", so he "passed it with some advisories" and it was collected by Jeanne.
He sent a text starting with a grin emoji which read: "Car is done. It's 560 you have to sort the seatbelt and the airbag light. I've passed the car though."
The seatbelts on the car had not ever been replaced and were defective.
Abigail Jackson, prosecuting, said she was not asking for the fatal crash to be treated as an "aggravating" factor to the offence.
Jon Lewis, defending, said Lewis had been in the motor industry for 30 years "with no prior criticism or complaint".
He said Lewis's decision to pass the MOT was "baffling" and it was unclear why he chose to ignore the standards "that had become the very foundation of his professional business".
Judge Charlotte Murphy said choosing the "harm" levels in the case was "not straightforward", saying she was able to consider the "wider impact" of the defendant's actions.
She said his actions represented a "dereliction of his duty" as an MOT tester but she accepted it was a "one-off offence" and "not motivated by financial gain".
Lewis was ordered to do 100 hours of unpaid work, and pay a £185 court surcharge and £114 costs.
The crash involved five people - Sophie Russon, Smith, Ross, Jeanne and Shane Loughlin, 32.
They were last seen at about 02:00 GMT on 4 March 2023 following a night out.
The families of the three young women had all reported them missing on the evening of 4 March, with Gwent Police receiving the first missing persons report in relation to the group at about 19:30.
Family and friends of the five made repeated appeals to find the missing group over the course of the weekend, with the mother of one of the girls saying she had been told to stop contacting police for updates.
They were eventually found just after midnight on 6 March.
The car had crashed deep in undergrowth near a roundabout, and they were not found for 46 hours.
Smith, Jeanne and Ross died while Russon and Loughlin were critically injured and taken to hospital.
In November 2025, the IOPC said four Gwent Police officers would face gross misconduct hearings.
Family members of those involved have previously expressed their upset and frustration over delays in getting answers about what happened that night.
In July, there were angry scenes outside the court when Lewis was confronted by the family of Darcy Ross. They showed his a picture of Darcy, telling him to "remember her name".
On Wednesday, none of the victims' families attended the hearing.