English to French Tech/Engineering Chemistry; Chem Sci/Eng for a company serving oil and gas industries

Hello dear participants!

I am translating a manual for a process and gas technology company, delivering as well caustic services.

This is the segment I have difficulty with:

"Check all control valves for proper fail positions" It applies to a kerosene treating unit, with a contactor, motor, pumps...

I want to understand FAIL position in this instance.

Thanks for your help.