Duty cycle is a matter of point of view. 80% on equals 20% off.

Key on engine off should be 70% federal and 85% Ca. There is no duty cycle for ignition. It is for KE.

O2 sensors don't do good statically. It is about impossible to read .4v. The difference between .25v and .75v is almost insignificant in fuel quantity. Get a curve of stoiciometry and see.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician