My O2 sensor is fitted to the manifold but I know some of the earlier systems had a sensor in the first bit of the down pipe.

I'm just thinking if your sensor was not removed carefully during the replacement of the catalytic converter then the wires might have been stretched so now you have a loose / dodgy connection - pure guess work on my part. Just thinking along the lines of what has been changed recently and the areas in which the work has been done. Do you have a spare good O2 sensor?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!