HFC Emissions fail

[Clairvoyant]

Hello fellow Z-ers,

 

I took the car for it's first MOT since I've had it and it passed on everything except for emissions.

 

It registered a CO of 0.44% (max allowed is 0.2%)

Here are some more details:
- DE engine
- Cobra HFCs (unknown installation date)
- I added Cataclean before the test and drove it for 40 mins
- The car had 97 or 99ron only over the last year
- It first recorded CO 0.72%, but after holding it at 3k rpm for 15-20 mins, it went down to 0.44 and wouldn't go any lower.

 

 

The garage said I need new cats. I've asked around and someone suggested new spark plugs and air filter.

 

So, my questions are:

- are spark plugs and air filter going to make up for the difference to make it compliant? I don't know when they were last changed.

- are new cats the only way to go? From searching on the forum, even new HFCs seem to be hit and miss.

- has anyone had any experience with the ISR Performance HFCs. They are the only ones I could find that are 300 cell. https://www.tarmacsportz.co.uk/isr-performance-nissan-350z-vq35de-03-06-high-flow-cat.html

 

Thanks