2003 Jazz with 290k km (180k miles).
Totally Failed at Emissions out of nowhere,

CO & Lambda all good, However CH at 740
After I changed spark plugs which looked totally fine,filters, taking it for a hard spin before the next test the CH was still too high at 240 and failed.

No errors on dash, local service couldn't see anything reported wrong when plugged to computer as well.
Any ideas where to look at and possible causes that could be causing this?

The only other thing that I think of is I started experiencing very very small almost non-noticeable mis-fires a couple year ago at idle ( all good when driving and there is no power loss or anything like that).

Lately, the exhaust also has started to sound like it's misfiring slightly.
I have had the car for 10years and the slightly weird sounds from the exhaust definitely are not normal.
Basically the exhaust should sound like:

prrrrrrrrrrrrrrrrrrrrrrrrrrrr
instead,it sounds like
prrrrrr puk prrrrrrrrrrrr puk-puk prrrrrrrrrr puk-puk-puk prrrrrrrrrrrr
Here is a video:

So in Short, CH Emissions are way too high, Car very slightly gives out a misfire sound at idle/ feels like the idle is more rough than it should be sometimes, exhaust sounds like it shouldn't.

Any ideas?
Thanks.

 

Have you done a compression test on it yet?

If low when dry, can try it wet with oil to help you figure out if it may be rings, or valves.