Once again, Carson online parts retailer U.S. Auto Parts Inc. has come under pressure from an investor to sell itself, but this time it’s coming from the company’s co-founder and former chief executive.

In a letter sent to the company’s board last week, Mehran Nia, who is also a director and owns nearly 12 percent of U.S. Auto Parts’ stock, says the retailer’s share price is habitually undervalued and that shareholders would be better served if the company explored putting itself up for sale.

He said he believes the company, shares of which trade for less than $2, should be valued at between $4 and $6, but that the company will keep trading below that value because it is too small to attract institutional investors.

“Given its relatively small market capitalization and limited trading liquidity, it is my opinion that US Auto Parts does not, and will not, benefit from continuing to be a publicly traded business,” Nia said in the letter.

In the letter, Nia said he has discussed the matter with potential buyers and is convinced there are a number that would be interested in purchasing the company “at a significant premium to its recent trading valuation.”

The letter seemed to catch investors’ eyes. Shares of U.S. Auto Parts rose 17 percent the day Nia’s letter was released, closing the week ended May 6 up 22 percent to $1.97. That made it the biggest gainer on the LABJ Stock Index. (See page 72.)

Nia declined further comment.

U.S. Auto Parts Chief Financial Officer Neil Watanabe did not return requests for comment.

This is not the first time an investor in U.S. Auto Parts has clamored for change.

Two years ago, activist investor Timothy Maguire, managing partner of Laguna Beach firm Maguire Asset Management, purchased a 5 percent stake in the company, then launched a bid to acquire the 28 percent stake of U.S. Auto Parts’ biggest shareholder, Oak Investment Partners of Norwalk, Conn. Maguire wanted to pressure the company’s board to bring in a new director, fire Chief Executive Shane Evangelist and cut costs.

Oak rejected the offer, saying it had confidence in the management team.

Maguire, which still retains his 5 percent stake, sent another letter to the board in June, urging the company to explore options to increase shareholder value.