I'd hate to fit a new catalytic converter only to be told later that it may be some lambda sensor

P0420 is most likely the catalytic converter no longer converting.

When the PCM sees the downstream O2 sensor voltage switching like the upstream sensor it sets the P0420 DTC.

 

Thanks for getting back to me!
I have been reading various threads on this subject to gain an understanding of the system.
I believe from my reading that the two sensors are either side of the front converter and it is likely to be this that has failed.
The vehicle is now 22 years old and has just passed 100,000 miles so I guess that it is not unreasonable for it to have expired.
At least it doesn't appear to be sophisticated to have any limp mode system to stop me driving it. That should allow me to continue to use it until I can sort out the new parts that I require.

 

Besides reading the code, have you done any further diagnostics, like checking the graph of the downstream O2 sensor..?

 

For what it is worth, my 99 (25yr old) with 266Kkm still running with the original converter and exhaust..

 

I'm not sure that my basic OBD2 scanner is sophisticated enough to do much more than just read the codes but I will check that tomorrow.
From your previous discussions that I have read I understand the basic concept that the upstream should vary whilst the downstream should be far more stable.
I take some comfort that your 99 vehicle has another 60,000+ more miles than mine as well. However, something has caused the discrepancy between the two sensors and thrown up the code.

 

Thank you all for your assistance with my previous problems which seems to have returned to normal over the last couple of months.
The only problem remaining is not bringing up a check engine light conditions but is annoying particularly as I am not having much success in identifying the cause of it.
This is on a 2002 Grand Vitara 1.6 and is a low engine speed (2000 - 2500 rpm) misfire as you open the throttle under load.


Back in my youth, when I worked in the motor industry and we had coil ignition with distributors and carburettors etc, we would occasionally get a misfire when in a high gear and opening the throttle wide, loading up the engine. This was usually a spark plug or HT lead breaking down.
Because of the similarity of the symptoms I changed the spark plugs as they were about due anyway. That didn't help so I changed the two HT leads which also didn't help.
I considered buying a coil and swapping it with each of the two originals to try to prove one defective but decided that an enquiry here may save me some time and money changing items without any certainty of success.
As this engine in fuel injected I did wonder if a defective throttle position sensor could cause a similar set of symptoms? I had a bad experience with one on my fuel injected Moto Guzzi California which has apparently caught out a lot of owners in the past.

I would be grateful for any suggestions or ideas that you may wish to share with me.
Thank you.

 

Well the parts cannon did not work, how about a scope on the primary and secondary of the ignition and post a wave form..

 

Unfortunately I no longer have access to much in the way of diagnostic equipment and so have to take a guess at possible causes and ask for the opinions of others who may have experienced similar symptoms. Some components are known to fail with a recognisable set of symptoms, like the cam position sensor that was identified and I was able to change recently, solving my problem.
Alternatively I may have to run with it until further symptoms develop aiding in the diagnosis. If all else fails I will take it to a local garage and get them to run some scans on it.
Thanks for your suggestion though.