This SAE Recommended Practice, limited to liquid coolant systems, establishes uniform cold weather bus vehicle heating system test procedures for all vehicles designed to transport 10 or more passengers. Required test equipment, facilities, and definitions are included. Defrosting and defogging procedures and requirements are established by SAE J381 which are hereby included by reference. This procedure is designed to provide bus manufacturers with a cost- effective, standardized test method to provide relative approximations of cold weather interior temperatures.