Help ~ Failed MOT on emissions

[Chris B N]

Hi guy,s
any help more than welcome
96 model NA1 Auto 59K miles
Just failed MOT on emissions
CO is OK on tickover but goes very high when revs increased
lamda high at tickover but reduces with revs

had new plugs in but made no difference
car appears to run very well did 370 miles over the weekend
(including keeping up with Simon)
any suggestions before replacing expensive parts like cats.
am having it checked on a tuner tomorrow.
Regards Chris B N

[Chris B N]

hi all,
think i'm sorted
got to get the parts ( Andy at VTEC sorting it)
the man with the computer says one o2 sensor not working at all,
the other one cannot make it's mind up what to do.
2 new sensors should fix it

BR Chris B N

[m666 edd]

I was going to write that it shouldn't be related to the cats as a byproduct they produce is co2. I assume if they weren't doing their job properly the co2 would be lower :)

Hope the sensors sort it.

[lotusolly]

Had exactly the same issues when having the MOT done one year. Emissions fine at higher revs but did not pass at the revs required for the test. I had a helpful inspector who just deleted that part of the test for me :) but was expecting a big bill for the following year.

Next year, took the car for a enthusiastic run before taking it for the MOT to make sure the cats were nice and hot and it sailed through the test. Seems the cats dont get hot enough at low revs or tickover for them to work effectively.

Olly

[m666 edd]

Did the O2 sensors sort this Chris?

I had my emissions tested and had the same problems.

[Chris B N]

waiting for delivery of the sensors today, fit and retest tomorrow.
at fast idle CO was 11.84 ( should be < 0.30) and HC 400 ( should be <200)
lamda reading was 0.687 ( should be in range 0.97- 1.03)
at natural idle CO was 0.51 ( <0.5)
will post the outcome and results after the retest.
Chris B N

[Chris B N]

New O2 sensors arrived Saturday, fitted today immediate pass on MOT emissions:)

as the old O2 sensors had stopped working should the engine warning light have stayed on to indicate a fault, which could then be identified using the self diagnosis?

[Nick Graves]

I understand that may not happen - the sensors' outputs go 'out of range' (IE they fib!) rather than them failing absolutely.

So the EMS doesn't 'know' there's anything wrong, because it cannot 'see' sooty exhausts or poor fuel consumption, which are often signs.

[Kaz-kzukNA1]

New O2 sensors arrived Saturday, fitted today immediate pass on MOT emissions:)

as the old O2 sensors had stopped working should the engine warning light have stayed on to indicate a fault, which could then be identified using the self diagnosis?

Hi, Chris. Your NSX is pre-OBD2 so only two O2 sensors used and they are before the CATs.
There are several conditions to trigger ECL (Engine Check Light) for faulty O2 sensors but unfortunately, it won't trigger it for the modest out of range signal.
In fact, ECU will try to adjust the fuel timer to compensate the deviation against the targeted A/F ratio and thus, causing strange hesitation without ECL from time to time.

Good to hear that you passed the MOT now. I also passed mine and the one on MX-5 just last week.

Regards,
Kaz

[Chris B N]

Hi Kaz/ Nick,
thanks the explanation, I wonder if the sensors have been faulty since I had the car (21 months) last year it failed emissions and as it sounded with an occasional miss fire, new plugs sorted this, but on retest only just passed the emissions. Also even with the new plugs the engine seemed to be running rich.
With the new O2 sensors engine seems smoother, sounds smoother and has
improved throttle response, all of which point to AF ratio now being better controlled.
All part of the ups and downs and pleasure of owning one of these cars.
See you at Silverstone
Chris