Help please! MX-5 NB 1.6 1998 failed MOT emissions on CO

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: __MX-5 NB Mk2 1.6 1998
  2. I’m based near: __Oxford
  3. I’m looking for technical help or recommendations on: __MOT Emissions Failure

Hi everyone. My MX-5 failed it’s MOT on emissions last week on CO and Lambda. I have replaced the O2 sensor with an IL Motorsports one as the Lambda reading was inconsistent. It’s been back for its re-test today and has failed again on CO, but HC and Lambda were good. It was serviced by a specialist in April, including air filter and plugs. It has no MOT fail history on emissions and last year’s test was an easy pass with very low numbers across the board. Since that test, the only major emissions work that has been done is a new aftermarket catalyst to replace one that was stolen. I’ve posted the emissions sheets from the two tests- Test 1 before O2 sensor change and Test 2 after. Grateful for any advice for what to look at next . Thanks.

Test 1
Test 1640×610 92.8 KB

Test 2
Test 2640×605 86.1 KB

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How often does the car get a good thrashing?

Perhaps you can persuade the tester to get the car hot, 78 degrees oil temp isn’t really up to full operating temperature.

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Thanks Adam - I’d wondered if it was warm enough. The car gets driven hard almost all of the time, although it is a weekend warrior. On the second test I made sure I took it for a 30 min thrash on the way to the test centre and the tester rolled it straight in. The fan was cutting in and out during the test and to be fair to the tester he had about half a dozen goes at getting it through. It looks like it will have to go to another garage for a fix if there’s no easy option so someone else will get the chance to be the MoT tester next time!

I’m leaning towards the cat as the numbers aren’t terrible - it’s not massively rich or lean and it would easily pass a non-cat test. It drives well, with no misfires or hesitation, and idles bang on at 825-850 rpm. I’ve heard that some of the cheap aftermarket ones don’t last.

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It’s interesting that when the Lamda reading is good the CO reading is bad, and when the Lamda reading is bad the CO reading is good.
I can’t help thinking this must be telling us something.
If it was a faulty CAT wouldn’t the CO readings be high all the time?
Will follow with interest as mine has just scraped through on CO when in previous years it has been 0.02.
D.

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Ha! Yes it’s more consistent since the O2 sensor was changed, just consistent that it fails on CO… It’s off to a garage for some diagnostics. I’ll post the answer when I know it!

When it passed last year, CO was 0.09, HC 28 and Lambda 1.01 at fast idle so its capable of much better.

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Hmm. Sounds like you’re miles ahead of me on that train of thought then!

Interesting point about the lambda / CO relationship, normally too much fuel (relative to oxygen, ie. rich) gives higher CO… the super lean test did the best so that makes sense.

Is it a tailpipe reading or OBD?

Final thoughts… (please don’t be offended as I’m sure you have these covered!), exhaust leak or blocked PCV?

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The readings were all from the MOT testers machine via the exhaust probe, so I don’t know what the car thinks it’s seeing.

It’s a good point about the PCV and exhaust - I hadn’t thought of those. I’ll have a look for anything obvious today and ask the garage to check them out :+1:

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All this seems familiar but more severe than my 2002 1800cc model. It passed MoT in Feb complete with new CAT and Lambda sensors but showed the engine warning light 50 miles later. I bought a diagnostic reader which lets me clear the codes and have kept record - the car shows the problem about every 27 miles after each reset, mostly after pottering type driving. Up the M6 it lasts about 50 miles so I wonder if it’s a time-based problem.
The garage reported the engine smelt rich but the emissions figures are OK. I wonder if the injectors are worn. Is it possible to get engine re-mapping to shorten the injector opening times?
Roger BF02KHZ

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So problem solved. The cat had failed, possibly as a result of the lambda sensor failing and the car running a bit rich for a while. The old cat was a cheap one, fitted when the original genuine one was stolen. The new one is also a cheap one, £80 ish from MX5 parts, as I didn’t want to gamble any more cash in case it wasn’t the problem. If it gives me another year then thats ok! Thanks for all your advice :grinning: New cat fitted and MOT passed.