94 XJS 4.0L 5-speed w/185,000mi getting FF23 OBD code. Smog check 1 year ago passed easily except for HC level at idle--barely passing. Last time I got the oil changed, check engine light went off for over 1000 miles. Back now with FF23 code.

Mechanic believes problem is with cat converter(s). Makes sense to me given the near failure on the smog test. Friend suggested I take the car to a muffler shop and have them cut out the bad cats and weld on some flanges so new cats can be easily added. I was also reading about cleaning the existing cats.

I have no direct experience with cat converters and smog check is required, again. Can anyone suggest a course of action?

 

Hmmm, I'm not sure that the cat would be first on my list of suspects but it is possible. Have you had someone put a sniffer in the tailpipe and measure actual emissions?

FF23 shows that the O2 sensor is reading rich. There can be a lot of causes for this. I've had it happen once on my car after a lot of spirited driving on a very hot day. The car did not want to restart easily and it threw the FF23 code. I reset the code (disconnect the battery) and it never came back on.

I would try a reset first, if it comes back on then things to check would be:
02 sensor: look for a lazy response and also check the sensor heater ground
Check the thermostat is not stuck open and that the CTS is reading correctly.
Check fuel pressure is not too high (faulty regulator)
Check purge valve is not stuck open

All these could cause an FF23 and to me are a bit more likely than a cat.
They are all pretty easy to check and a LOT cheaper to repair than a cat

 

Thank you for responding, Allan.

Other than the smog check I mentioned, no.

I've already reset a number of times. O2 sensor is new. Previous owner changed it recently. I was getting FF44 until I had it adjusted correctly. Now getting the FF23. Mechanic says sensor is working fine but may be the wrong sensor giving faulty reading. No way to check without pulling it out.

Don't know how to check fuel pressure or purge valve.

My reasoning about the cat was I've seen many posts of people that eliminated FF23 by changing the it, my mechanic suggested it, and the high HC during idle on the last smog check. Since the code went away for 1000 miles after the last oil change, it seemed reasonable this might be the problem.

You've convinced me to look elsewhere, though. I have to get new tires and have a small valve cover leak to fix so I'm going to have my mechanic do all that and change the oil. Perhaps it will clear the code and pass the smog check.

Cheers!