There can be no closed loop with the EHA disconnected. I believe the duty cycle should react to the fixed mixture and walk off the scale trying to correct.

The way to calibrate your reading with duty cycle is to disconnect the O2 sensor. The duty cycle should read exactly 50% then. If it is anything else you have either a testing (intrument or connection) problem or a system problem. It is ever so important that one calibrates ones testing before drawing conclusions.

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