A fault diagnostic method for an automatic transmission, the method comprising: monitoring an operating state of a trim system configured to selectively supply clutch engagement pressure and exhaust to at least one clutch control valve (162, 96), the trim system comprising: a first trim valve having: a first operating state in which the first trim valve supplies a clutch engagement pressure to at least one clutch control valve via a first fluid passageway, and a second operating state in which the first trim valve exhausts the first fluid passageway; a second trim valve having: a first operating state in which the second trim valve supplies the clutch engagement pressure to the at least one clutch control valve via a second fluid passageway, and a second operating state in which the second trim valve exhausts the second fluid passageway, and a first pressure switch fluidly coupled to one of the first and second trim valves such that the first pressure switch is in a first operating state when the first and second trim valves both supply the clutch engagement pressure to the first and second fluid passageways respectively and also when the first and second trim valves both exhaust the first and second fluid passageways respectively, and is otherwise in a second operating state different from the first operating state; determining an expected operating state of the trim system based on current operating conditions of the transmission, and generating a fault signal if the monitored operating state of the trim system is different from the expected operating state of the trim system. monitoring an operating state of a trim system configured to selectively supply clutch engagement pressure and exhaust to at least one clutch control valve (162, 96), the trim system comprising: a first trim valve having: a first operating state in which the first trim valve supplies a clutch engagement pressure to at least one clutch control valve via a first fluid passageway, and a second operating state in which the first trim valve exhausts the first fluid passageway; a second trim valve having: a first operating state in which the second trim valve supplies the clutch engagement pressure to the at least one clutch control valve via a second fluid passageway, and a second operating state in which the second trim valve exhausts the second fluid passageway, and a first pressure switch fluidly coupled to one of the first and second trim valves such that the first pressure switch is in a first operating state when the first and second trim valves both supply the clutch engagement pressure to the first and second fluid passageways respectively and also when the first and second trim valves both exhaust the first and second fluid passageways respectively, and is otherwise in a second operating state different from the first operating state; a first trim valve having: a first operating state in which the first trim valve supplies a clutch engagement pressure to at least one clutch control valve via a first fluid passageway, and a second operating state in which the first trim valve exhausts the first fluid passageway; a first operating state in which the first trim valve supplies a clutch engagement pressure to at least one clutch control valve via a first fluid passageway, and a second operating state in which the first trim valve exhausts the first fluid passageway; a second trim valve having: a first operating state in which the second trim valve supplies the clutch engagement pressure to the at least one clutch control valve via a second fluid passageway, and a second operating state in which the second trim valve exhausts the second fluid passageway, and a first operating state in which the second trim valve supplies the clutch engagement pressure to the at least one clutch control valve via a second fluid passageway, and a second operating state in which the second trim valve exhausts the second fluid passageway, and a first pressure switch fluidly coupled to one of the first and second trim valves such that the first pressure switch is in a first operating state when the first and second trim valves both supply the clutch engagement pressure to the first and second fluid passageways respectively and also when the first and second trim valves both exhaust the first and second fluid passageways respectively, and is otherwise in a second operating state different from the first operating state; determining an expected operating state of the trim system based on current operating conditions of the transmission, and generating a fault signal if the monitored operating state of the trim system is different from the expected operating state of the trim system.