FAA has issued AD 2018-02-04 Aerospace Welding Minneapolis, Inc. applicable on Muffle installed on Cessna 172R/S. Effective Date of this AD is February 21, 2018Muffler part No. A1754001-23 172R S/Ns 80001 and up; 172S S/Ns 8001 and up Muffler part No. A1754001-25 172R S/Ns 80001 and up; 172S S/Ns 8001 and up; This AD was prompted by occurrences of cracks or broken welds in the connecting weld of the muffler body to muffler cuff that may allow carbon monoxide (CO) exhaust fumes into the cockpit heating system. We are issuing this AD to prevent cracks in the connecting weld of the muffler body to muffler cuff that may allow CO fumes to enter the cockpit heating system and possibly inhibit the pilot’s ability to maintain control of the airplane. Compliance :Comply with this AD within the compliance times specified, unless already done.Inspection of the Muffler (1) Within 5 hours time-in-service after February 21, 2018, inspect the affected muffler following the instructions listed in paragraphs (1)(i) through (iii). (i) Using a vacuum cleaner with the hose attached to the blowing side of the vacuum (with the filter installed), attach the vacuum to the airplane tailpipe and seal securely. (ii) The vacuum will pressurize the system sufficiently for a soap solution to be brushed or applied from a spray bottle to the surface of the exhaust system (iii) Inspect for evidence of breaches (leakage) in the system from cracks. (2) In lieu of doing the inspection required in paragraph (1) , within 5 hours after February 21, 2018, you may remove and replace the affected muffler. (3) If replacement specified in paragraph (2) is done instead of the inspection required in paragraph (1) , then paragraph (3) is the only additional requirement. Replacement of the Muffler (1) If evidence of breaches (leakage) is found during the inspection required , before further flight, remove the affected muffler following AWI Cessna 172 (Lycoming) Muffler Removal and Installation, Revision 01, January 17, 2017, and replace the affected muffle (2) If no evidence of breaches (leakage) is found during the inspection required of this AD, within the next 100 hours TIS after February 21, 2018 or at the next annual inspection, whichever occurs later, remove. (3) After February 21, 2018, do not install on any airplane an affected muffler.