I would suggest calibrating your tool.

There is a very simple way. Disconnect the O2 sensor. Start the engine and you will be reading 50%. If you aren't fix that first. I'm not sure what you are reading but I'm pretty sure it isn't closed loop control.

You actually don't even need to start the car but the fuel pump relay has to be jumped otherwise as lambda control is taken off one terminal of the FP relay.

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