What was found
(CAN) LEAK DETECTED. UPON COMPLETION OF AD 90-03 R2 LEAK IDENTIFIED IN CABIN HEAT EXCHANGER. UNIT IS A FAA-STC AFTERMARKET EXHAUST INSTALLED PER STC SA01801AT. SIMILAR INSTALLATIONS CONTAINING THE SAME DESIGN ARE INSTALLED IN MANY OTHER MODEL AIRCRAFT. IT CONTAINS A UNCONVENTIONAL DESIGN NOT IDENTIFIED IN FAA AC 91-59A (INSPECTION AND CARE OF GENERAL AVIATION EXHAUST SYSTEMS). LEAK OCCURRED IN AREA DIFFICULT TO VISUALLY INSPECT AND REPAIR. ICAW FOR THE EXHAUST INDICATES A 100 HOUR REPETITIVE INSPECTION INCLUDING SUBMERGE PRESSURE TEST. HOWEVER THIS INSPECTION IS NOT REGARDED AS MANDATORY BY TYPICAL USERS. THIS IS THE 5TH TIME A CRACK HAD APPEARED IN THE EXACT SAME AREA. UNIT REPAIRED PER AC43.13 AND RETURNED TO SERVICE.