The foreign secretary, who is the frontrunner for next prime minister, has hinted she would like to stop the privileges committee probe into whether Mr Johnson misled parliament.
She raised eyebrows on Tuesday evening after saying she she would in principle vote to cancel the investigation.
And she and Rishi Sunak are yet to say whether they keep Mr Johnson in front line politics, or give him a peerage or knighthood.
In a letter to Ms Truss Liberal Democrat Cabinet Office spokesperson Christine Jardine urged the likely next PM to rule out coming to Mr Johnson’s rescue.
And she said it was vital that the next PM “commits to restoring standards in public life” by ruling out gongs Mr Johnson, among other pledges.
The opposition party also said Ms Truss should put on record that she would resign if she lied to parliament.
And the liberals want the ministerial code to be written into law, with a new independent advisor to oversee it.
“During his three years as Prime Minister, Boris Johnson has comprehensively trashed standards in public life,” the letter says.
“From the highly concerning decisions regarding the appointment of Chris Pincher to the Prime Minister’s lies over partygate; from the Owen Paterson affair to the Downing Street flat renovation, we have seen scandal after scandal engulf the Government.
“This has all done enormous damage to public trust in politics. For the good of our democracy, it is essential that we seize the moment of the Prime Minister’s departure to restore that trust and make politics work for the British people once again.”
Ms Truss and her campaign have yet to respond to the letter. When Mr Johnson was forced from office last month Ms Truss was notable among ministers for saying she backed his leadership.
Mr Johnson himself has yet to say whether he will seek a new government role, or even stay as an MP – but his political career could be given an indefinite extension if Ms Truss gives him a life seat in the House of Lords.
He has reportedly told aides that he could make a comeback, and even become prime minister again – in an apparent attempt to emulate Winston Churchill.
“The public have lost all trust in Conservative politicians. In scandal after scandal they have proven themselves to be the party of lies, law-breaking, and corruption,” Lib Dem constitution spokesperson Ms Jardine said.
“Both Truss and Sunak propped Boris Johnson up for months, and Truss’s comments are just the latest attempt to bend the rules and let Johnson get away with his disgraceful behaviour. He is clearly unfit to hold public office, let alone receive any kind of honour or peerage.
“We cannot have an Owen Paterson 2.0. As trust in politicians is at an all time low, it is vital that the Conservatives clean up their act.
“Mending our broken politics must be a priority for the next Prime Minister. The Conservative Party is so submerged in sleaze that it has been unable to do anything to address the NHS crisis, the transport chaos or the cost of living emergency.”