If the microwave exhaust fan is not circulating air properly, there could be an issue with the grease filter. A dirty filter will cause your microwave to run less efficiently and can alter the taste or smell of your food. Filters should be changed every six months for maximum performance. These silver, aluminum filters are sold separately, and are approximately 13 3/8 inches long x 5 7/8 inches wide. Measure your current filter to assure correct replacement. Check your grease filters regularly for particles and grease buildup to see if cleaning is necessary. If there is any visible damage to the filter, it should be replaced straight away.