The parliamentary committee was launched after VW – Europe's largest carmaker with 12 brands ranging from luxury Porsche to lower-end Skoda – admitted in September 2015 to fitting some 11 million diesel vehicles worldwide with software that temporarily reduces harmful emissions when undergoing regulatory tests.
Tasked with investigating Berlin's response to emissions excesses going back to 2007, the group of lawmakers have already heard witnesses from across the government and Volkswagen, including the car group's former chief executive Martin Winterkorn.
Merkel has been chancellor for the entire period covered by the inquiry, which has also heard serving and former ministers including Foreign Minister Sigmar Gabriel, who previously served as economy minister, and Chancellery chief of staff Peter Altmaier.
The hearings have yet to uncover clear evidence that Merkel or her government acted improperly.
Her appearance before the emissions committee is Merkel's second as a witness this year, coming less than a month after she deflected MPs' questions about mass spying by Germany's BND foreign intelligence service on behalf of the US National Security Agency.
Battery of questions
When Merkel takes her seat as the inquiry's final witness, MPs will come at her from a number of angles.
One of the top questions is why the diesel cheating was uncovered by officials in the United States, rather than Germany's own KBA motor vehicle authority.
California's powerful Air Resources Board (CARB) was one of the first regulators to bring the VW scandal to light, along with the US Environmental Protection Agency.
Transport minister Alexander Dobrindt – whose brief includes oversight of the KBA – told the inquiry in his own testimony that the regulator “acted faster than others” in responding to the scandal.
MPs also want to find out more about a conversation between Merkel and then-California governor Arnold Schwarzenegger in 2010.
Merkel criticised strict regulations on diesel vehicles in the most populous US state, CARB chief Mary Nichols, who was present at the meeting, told the inquiry Monday.
Nichols was “surprised” that a head of government was informed in such detail about the topic, according to a summary of her testimony published on the Bundestag (German parliament) website.
Closer to home, the lawmakers have questions about whether Merkel intervened in Brussels to de-fang European emissions regulations for the benefit of German manufacturers.
And they will be attentive for any hint that Merkel might have known earlier than she has so far acknowledged about the scandal.
Former VW CEO Winterkorn told the inquiry that Merkel learned about 'dieselgate' in a phone call on September 22, 2015 — just after Winterkorn himself claims to have found out.
But prosecutors are already investigating the carmaker's one-time “Mr. Quality” for fraud, saying they have “sufficient indications” that he may have known about the questionable software earlier than he has admitted and failed to inform investors.
In the United States, the FBI believes senior VW executives in Germany found out months earlier than they have so far claimed, in July 2015.
That allegation could cost VW dear in Germany, where investors are suing the firm for billions over what they say was a failure by executives to inform them of the scandal in a timely fashion.
VW has so far set aside 22.6 billion euros ($23.8 billion) to cover fines and compensation related to the affair, but experts estimate the final bill could be much higher.
Under government's wing
German politicians have long nurtured the nation's auto industry, one of the country's strongest and an employer of some 800,000 people.
Volkswagen's deep links with the Social-Democrat led Lower Saxony government, a major shareholder with seats on the firm's board and ties to its powerful works council, have often been criticised.
And Merkel was quick to intervene last month when France's Peugeot said it would buy German carmaker Opel, helping secure guarantees that the new owners would uphold collective-bargaining agreements with workers in personal talks with chief executive Carlos Tavares.
The task of lawmakers on the inquiry committee is to determine whether such feather-bedding for the favoured industry extended as far as turning a blind eye to much higher emissions of harmful nitrogen oxides than allowed.