Pressurization Auto fail and worning sound.

[shaun72]

Good day .
We had on our b737-500
While fly climbing to 35000 ft an auto fail light and a warning sound. We were not able to control the cabin with auto and standby modes. We had to fly Manual.
My question why the controller did not transfer to standby? No evidence of door seals leaking, duct pressure was normal a did test the control panel p5 works ok.please can some one help?