During my last 3 MOT I have been advised that because of excess emmisions I needed a new cat. In 2018 I bit the bullet and bought a new one. In 2019 I had the same result and the cat was replaces foc. 2 weeks ago, same result, but supplier would not do the same again as they said the fault was probably elsewhere.
Took to a main dealer for a diagnostic - no fault codes found in the system but agreed emissions were high. They commented that the original lamda sensor was still present i.e it was apparentally not changed when the cat was replaced the year before.
Now faced with replacing the cat again and crossing my fingers that it would pass MOT. Any thoughts please?
You’ve not said what model & year, this will help with diagnosis
Its either you’ve been really unlucky with cats or your lambda sensor and or maf sensor are giving false readings. This happened to me last year (cat had been replaced 2 years earlier). No codes just high hc and Co readings on the machine. New lamda and maf sorted it. Mine is a mk3 fwiw
@EXCEL_fugitive, the first question I have to ask before anything else; has the car got any evidence of oil consumption?
Can you state the model, year and mileage?
On the same note, when did it last have an oil change, air filter and spark plugs?
What’s the fuel economy like?
I don’t know if this maybe of some help.
I had a similar problem with my Mk2.
Last year fitted it with a K&N air filter, new leads, iridium plugs and changed buying petrol from supermarkets to using Shell 99 octane.
This year no emission problems.
Michael
As a thought… in passing…
High emissions can be cause by mechanical wear in the engine and changing the cat and other peripherals will not solve the core issue. It may solve the emissions enough to pass the MoT test that year but recurring failures indicate a deeper problem.
Try a basic test like compression, (wet and dry tests are a good indicator) or injector spray pattern.
If you have one cylinder greater than 15% - 20% different (not sure that Mazda state as the limit for their design) to the others this will reflect in the engine sensors and affect the PCM (ECU) fuelling and cause excess fuel to be injected causing high emissions. The engine runs fine but the delicate balance of modern engines tuning and emissions will be affected.
If you can log and study the MAP (Absolute Pressure) sensor for excessive variation this is another indicator of wear in the engine.
Thanks BBB
Thanks aaa53
Thanks Michael - roughly how much did that cost you for parts - thanks for your interest. David
I think the whole lot came to about £150.00 in total.
Over the past year I have noted cleaner and more responsive acceleration.
I put down to the parts fitted and the use of Shell V-Power fuel.
Michael.
I took my 2008 NC 2.0 Sport for its MoT in March and it failed on emissions. I anticipated having to look at a replacement CAT, but opted instead for a s/h back-box as I suspected the other had a minute leak, as the exhaust noise did not sound quite right. In addition I almost emptied the fuel tank, before re-filling with Shell’s ‘super’ fuel and a bottle of Cataclean.
Spent a week revving the engine more than I normally would, as advised by the MoT tester, as it is a relatively short trip to where I work, and I had not had much opportunity to do longer runs. I was therefore very pleased to see the green numbers come up on the retest, and not have to worry about any further diagnostic costs.