Volkswagen questioned by Parliament
MPs got their chance to grill UK representatives for Volkswagen and Transport Secretary Patrick McLoughlin earlier.
Details are still emerging about what the company does and does not know about what went wrong and how it plans to fix the situation.
Here is what we learned.
Which cars need new parts?
Out of the 1.2 million UK motors affected, about 400,000 will need fuel injectors modified, as well as a software fix, said Paul Willis, managing director for Volkswagen's UK operation.
These are vehicles with 1.6-litre engines. Cars with 2.0-litre engines will just need to have their software updated.
Will these fixes affect my fuel efficiency?
"The brief the engineers are working to is there can not be a change to the miles per gallon," Mr Willis told Parliament. "One of the key objectives [is that it] maintains the same level of fuel consumption."
What is being done now? What should customers do?
Volkswagen are writing to owners of affected cars ands have set up a website so owners can see if their car will need to be fixed.
Engines should be fixed by the end of next year, although there is "some risk" of that target being missed, said Mr Willis.
"The goal is to try to get it finished by the end of 2016," he said.
"Where a customer is inconvenienced, we will have to provide a loan car," he added. When asked about the potential for compensating customers over lost value to their vehicles, he said, "I think it's premature to think about that."
He pointed to safety scandals in the US, which have included airbag and ignition problems, and said those cars were recalled and fixed with no loss of value.
For how long have cars that could have cheated tests been sold?
The first VW car with a so-called defeat device in the UK was sold "around 2008," said Mr Willis.
But it wasn't until 19 September 2015 that he heard about the problem in the US, he said. And the company moved to cease sales of the affected cars once his firm knew precisely which ones were affected.
The combination included 60 different models, five different brands, three different engines and two different transmissions, he said.
However, the eight-day gap meant more than 1,000 cars were sold in the UK after authorities knew Britain was affected.
Has VW seen a drop in sales as a result?
Sales and orders for VW vehicles were down "a little bit" since the scandal broke, said Mr Willis, but the company's other brands are selling as normal.
Transport Secretary Patrick McLoughlin, who was also being questioned, told the committee that VW "have behaved in an appalling way".
I'm a Volkswagen dealer. How will this affect me?
"The cars selling today are EU6 cars that are not affected," said Mr Willis. He said VW will pay customer satisfaction bonuses for dealers and the firm has not charged distributors the stocking costs for affected cars.
Will Volkswagen be fined in the UK?
"At this stage, we are at the very early stages of finding what has gone on," said Transport Secretary Mr McLoughlin. But, he warned, under the right circumstances, "the secretary of state can prosecute."
The devices being used to cheat emissions tests had been made illegal in 1998, he said.
"It is fairly unbelievable to think that a company of the size and reputation of VW has been doing something like this and finding ways around regulations.
"I think they are going to suffer very substantial damage as a result of it and they deserve to, quite honestly."