Dear OwenF,

Hope you're well. The first step is to see what codes are found in the On-Board Diagnostic (OBD) system. See this site's FAQs for instructions as to how to obtain any OBD fault codes.

Some fault codes are very specific, others not. For those, that are specific - e.g., Oxygen sensor's signal missing" - check the wiring the the sensor itself. If the sensor is factory-original, then replace it. In a car that is 27 years old, the factory-installed sensor doesn't owe You anything.

The soot coating on plugs suggests - as you note - a "rich condition". That could result from a failed engine temperature sensor. If the engine control unit thinks that the engine is "cold", it will keep the mixture rich for far longer than is needed (or helpful). Again, if that sensor is the factory-original item, it is long past the end of its service life.

The spark plugs can be easily replaced and likely should be, if they're coated with baked-on carbon deposits.

Correction of the underlying problem(s) could result in the catalytic converter clearing any soot by "burn-off". Thus, I'd not rush to replace the catalytic converter. The factory-installed units are robust and replacements are not cheap.

Hope this helps.

Yours faithfully,

Spook