Akhtar and three other players appeared before a four-member medical commission formed to assess the spate of injuries during Pakistan's 2-1 Test series loss to India in March-April.
But Akhtar's injury, sustained after he fell in his follow through in the decisive Test at Rawalpindi, was seen as suspicious and was particularly inquired.
"After the game, on 14th April 2004, the MRI scans showed no injury and subsequently other bone scans were done in Lahore which did show intense stress but the commission found the injury inconclusive," said PCB chairman Shaharyar Khan at a press conference in Lahore.
Pakistan team members were also cautioned to show more commitment on the field. "We have not panicked after the defeat but the manner of abject surrender in the last Test was disgusting," said Khan.
Pakistan lost by an innings and 131 runs on the fourth morning, handing India their first ever Test series win in Pakistan.
"Subsequent scans on April 28 revealed damage around the 11th rib but there was no conclusive evidence that the injury was sustained during the Test or Akhtar was carrying an old injury."
"The PCB has accepted the player's (Akhtar) own contention at face value that he was in pain and could not take the field on the final two days of the Rawalpindi Test," said Khan.
Akhtar said, "I am disappointed (with PCB's ruling) but at the same time I will continue to give my best for the country." Khan defended team captain Inzamam, coach Javed Miandad and manager Haroon Rasheed.
"They (the management) tried to handle Akhtar but he maintained that he was injured," said the PCB chief.
The medical commission has strongly recommended no sporting activity for four weeks and a repeat of bone scan, MRI scan and CT scan after four weeks of the first scans for further evaluations.
The PCB has asked Akhtar to have more tests on May 26 but the pacer conveyed to the Board that he was leaving for England tomorrow to play for the Durham county and would send the reports from there.
However, the PCB found Akhtar's conduct as "unacceptable".
"The Board has taken a serious note of Akhtar's conduct after he left the field which according to the doctors and team management was demoralizing to the team morale," said Khan.
"Accordingly, the Board has decided not to invoke any penalty against Akhtar but he has been warned and will not be, in future, be given any special treatment."
Doubts were raised over Akhtar's injury by captain Inzamam-ul-Haq after he batted on day four without showing any signs of discomfort.
Akhtar, seen as Pakistan's main weapon, managed a disappointing seven wickets in the series.
"Other players have also been cautioned that they need to demonstrate 150 percent commitment to team spirit to merit selection in future," he said.
Pakistan's next engagement is a three-match One-day series against Sri Lanka in Malaysia, still to be confirmed, and then the Asia Cup in Sri Lanka in July where they once again came face to face with India.