Emissions failure - very confusing

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 2011 MK3.5 (NC) 2.0 Sport Tech (Retractable Hard Top), 138,000 miles
  2. I’m based near: Hertfordshire
  3. I’m looking for technical help or recommendations on: Emissions too high/intermittent engine light

Hi all, I have a really confusing issue that I am really struggling to sort. The car has been to multiple garages without a concrete diagnosis. Wanting any advice/input from anyone, or to help brainstorm solutions!

The main symptom is MOT emissions - I had an MOT that the car was put through as a pass (but I was informed that the emissions were not pass-able - the car was put through anyway). To my eye/ear/feeling, the car idles rough, and shakes more than it should, and stalls easier that I would like. But I have not had an MX-5 before, so don’t know if it is entirely normal.

I took to a garage, they hooked up tester and confirmed ‘emissions too high.’ They latched on to an old engine light that had come on briefly a couple of months before for a day or so. It was code P2096, which I have looked in to extensively. This has many possible causes, but this particular garage ruled out compression issues, or air leaks. They were convinced the P2096 means the Air Fuel ratio sensor (also known as the first oxygen sensor) needed replacing. Replaced with new aftermarket sensor, same engine light (P2096) now permanently on with new sensor, but interestingly after it drove very well for about 3 or 4 miles. They then determined it needed a genuine Mazda sensor, as the car didn’t like the aftermarket sensor. This didn’t cure problem, and they gave up.

Original air fuel ratio sensor put back in, no more engine light, but emissions still high. I took the car to a well regarded MX-5 specialist, who couldn’t use engine light in diagnosis (testing car as presented, and the historic code had been cleared), but agreed emissions still high: Lambda: fine, Hydrocarbons: slightly high, CO: Near fail. Advice was to drive the car, but monitor oil levels to see if it burns oil.

Again, the P2096 code has been very rare with the original sensors in place (come on twice in 4 months, for a day at a time).

Not expecting an immediate light bulb moment, but any advice would be much appreciated because it is completely spoiling any enjoyment I get out of the car - I don’t want to drive it, and to be honest it’s making me want to give up and get rid.

Thanks in advance!

2

What does the temperature gauge show?
If the thermostat is stuck open and the car is running cool, the mixture may be rich to compensate

3

Thanks, that’s a good suggestion. To the best of my knowledge, temp has always been ok. Cool at start, takes a few mins but rises up to kind of midway up the gauge. Believe it’s normal?

4

Higher miles, are the spark plugs ok? Changes usually around every 64k miles or thereabouts,might be worth a try. Have you service history? Guessing a this may have been gone through with the MX-5 specialist?

5

Another good idea - might have a look at spark plugs. Like you say, I would expect specialist did look at them.

They were changed about 1000 miles ago just after I bought the car, but the same mechanic that did them also only did the diff drain plug up finger tight, so not completely out of the realms of possibility!

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6

Are you aware of it burning oil?
Contamination is a potential factor as well.
I’d imagine pulling the new plugs for inspection( cough?) may help to delete my suggestion if nothing else.

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7

I think I might just need to drive it for a while - I haven’t done a huge amount of miles since purchase, so will keep track from now on.

8

Are they the correct spark plugs for the 2.0 variant? There have been a few instances of incorrect plugs being fitted with the result that the car runs poorly or fails emission test. The 1.8 plugs for some reason are completely different but look very similar and will fit, plus some suppliers tables are apparently showing the wrong plug types for your engine.

9

Thanks for the suggestion - spark plugs are now next on my list of things to test/check.

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