A top executive at United Rentals Inc., the world’s largest equipment rental company, has less than a month to explain questionable accounting. The ultimatum came almost one year after the firm said it was "cooperating" with a U.S. Securities and Exchange Commission probe.

John N. Milne, president and CFO of the Greenwich, Conn.-based firm, "failed to perform his duties" when he refused to answer recent boardroom questions, says the company in a statement. A special board committee had found possible evidence that equipment sale-leaseback transactions were wrongfully posted as profit.

United says the clues "suggest" that the accounting "for at least some of these transactions" was incorrect. Directors questioned up to 40 employees, including Milne, who refused to talk, according to Chuck Wessendorf, company spokesman. The board gave Milne 30 days or face termination. Wessendorf says the special committee has reached "no final conclusions" as yet.

Amid an ongoing SEC inquiry, United plans to restate its finances from 2000 to 2004. Changes to reported self-insurance reserves are expected to raise pre-tax income for 2003 and 2004 but lower it for earlier years. The firm estimates 2004 revenue at $3.1 billion and expects $3.4 billion this year.

United operates in the U.S., Canada and Mexico. It owns a fleet of construction equipment costing $3.7 billion. Bolstered by higher nonresidential activity this year, United’s first quarter rental rates shot up 9.7% over a year earlier. Overall, the firm expects rates to average 5% higher this year.

Milne, who also is the firm’s chief acquisition officer, founded United Rentals in 1997 with board Chairman Bradley S. Jacobs.