Obviously the feedback controlled Lambda system isn't working. Six percent CO is terrible. It would be unthinkable coming from the engine, let alone coming out the tailpipe after the cat.

The system is based upon the o2 sensor and proper set up. There are basically two different types of sensors and I presume the Ford one you chose was of the right type. Its probably not the sensors fault. The first way to test it would be to disconnect it and test the exhaust. If the sensor is grounded it could drive the mixture at the outlet to maybe 6% but the basic mixture setting (this car must have this done properly, the system doesn't do it like the adapting systems built after CIS). The system is usually adjusted electronically (see the DIY article on evaluating engine controls) but since the O2 sensor is fouled at least here, one must get back to basics.

I would disconnect the sensor, set the mixture with an exhaust gas tester to 0.5% to 1.0% CO, drive the car hard, retest and then reconnect the O2 sensor and see what happens. It can be watched electrically for evaluation (see the article).

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