MX5 NB - CO emissions too high at MOT despite new cat

[lobster940]

1999 Mazda MX-5 NB 1.6 here.

I've replaced my catalytic converter (DIY on my driveway), after the car previously failed an MOT test on CO emissions. CO emissions are still running too high.

What could be the issue here? Lambda sensor needs replacing?

The car isn't driven much thanks to ULEZ - garage ummed and ahhed and simply suggested taking the car for a good run. Not sure this would bring down the CO emissions sufficiently though?

Any thoughts on how I could sort this?

Thanks!

[lobster940]

I should mention - I fitted the new catalytic converter then just drove the car 50 metres down the road to the MOT station.

Do I need to 'bed in' this new catalytic converter - by giving the car a good run - for it to start working effectively?

[Wacky Racer]

My low mileage (25,000) miles Mk 3 failed about three years ago, (EGR valve)

They tried to clean it without success and had to fit a new one,

[Belle427]

Cats do need to be hot to work effectively, especially aftermarket cheap ones.
High CO suggests too much fuel.
Lambda reading suggests you may be seeing a slight air leak in the exhaust?

Edited by Belle427 on Friday 26th July 13:24

[lobster940]

Wacky Racer said:

My low mileage (25,000) miles Mk 3 failed about three years ago, (EGR valve)

They tried to clean it without success and had to fit a new one,

Was a faulty EGR valve specifically causing high CO emissions?

[lobster940]

Belle427 said:

Cats do need to be hot to work effectively, especially aftermarket cheap ones.

OK - and yes it is a cheap aftermarket cat - I will give it a good hard run before I present it at the MOT station.

Belle427 said:

High CO suggests too much fuel.

Assume you mean it's running too rich? How to address this? Clean out the MAF? Replace the lambda sensor (are there two of these)? Or just thrash the car and hope that a hot aftermarket cat = a working cat?

Belle427 said:

Lambda reading suggests you may be seeing a slight air leak in the exhaust?

This'll likely be from the join between the rear of the new catalytic converter and the exhaust, where I didn't apply enough exhaust sealant paste.

Edited by Belle427 on Friday 26th July 13:24[/footnote]

[footnote]Edited by lobster940 on Friday 26th July 15:00

[Belle427]

I'd try getting it hot first and re testing. Don't know how much time your Mot guy has for you regarding the emissions test.

[lobster940]

100 miles of..."unusual" motorway speeds in 4th gear...and the 54 quid eBay cat decided to wake up!

Should probably actually invest in a better cat, but it's through the MOT, and I can take it down to France for the summer, which is all that matters now.

[Belle427]

Well done, a pass is a pass as they say!