At 1100 Pacific time, a Cessna 172M porpoised on landing at Catalina Island Airport, damaging the firewall. The pilot and three passengers were not injured. The pilot said the approach was normal. The flap position indicator ceased to function during the flight and the pilot estimated that he used 20 degrees of flaps for the approach and landing. He said that once he had the runway made, he reduced the throttle to idle and flared. The engine continued to produce power, running at 1,200 to 1,600 rpm and the airplane bounced several times as he tried to get it to settle on the runway. Finally, he pulled the mixture to kill the engine. The engine had accumulated only 17 hours of operation.