NA Lambda sensor MOT issue

  1. My model of MX-5 is: __NA Eunos 1993
  2. I’m based near: Harlow Essex_
  3. I’m looking for technical help or recommendations on: __MOT fail on emissions and lambda sensor

Dreaded MOT fail on emissions. Even no cat emissions. Garage told me that there is no lambda sensor on the exhaust and so the ECU is mixing the fuel wrong and hence terrible emissions.

Thinking of getting a new IL motorsport exhaust but not sure these come with the lambda, so I’d still have the same problem.

Anyone have experience of this problem?

What you removed the lambda sensor? Surely you have terrible fuel consumption.

Never heard of a NA exhaust manifold having no lambda fitting.

I suspect the garage is clueless, and thinks a Mk1 will have two lambda sensors, one after the cat. You only have one. Like this one has:


They were probably thinking of the post-cat sensor on JDM cars that’s famous for throwing up a permanent HEAT warning light when it burns out. It looks like a lambda sensor to the untrained eye.

Yes I think you’re right. No rear lamda sensors for a NA on eBay for example - but there is wiring back there. Seems there no front sensor either in mine as far as I can see.

A friend has suggested I play with the timing settings of the engine since this won’t be getting adjusted with no lambda.

I’ve got a lambda hole, but it has a lambda plug, no sensor. Previous owner had removed it. However I’ve heard from MX5 friends who also have their sensors removed or disconnected. Track focused cars though so fuel consumption not so much of a priority.

I really think that you need to establish what is and isn’t fitted to your car before starting any diagnosis. Can you post a close up photo of the electrical connectors at the back of the cam cover?

But mechanics have trained eyes. And the sensor isn’t post cat, its on the cat.

Find another mechanic if this is the extent of their problem solving.

To extinguish the HEAT light, disconnect the sensor. I have no such sensor on my Eunos Roadster. It was deleted about 15 years ago.

I suppose in theory the car can run “open loop” - people sometimes fit carburettors, for goodness’ sake - but fuelling is always going to be about the ECU looking at readings and guessing how much should go in, without knowing whether it’s actually burning properly. Engine wear and condition will alter how much fuel it actually needs for optimal combustion.

Update. There IS a lambda near the manifold. But not post cat. My confusion was that on the first pipe after the manifold there was a lambda hole but it is bunged. The lambda wiring connection from the manifold looked shoddy so a friend has upgraded it and things seem to be better. Will get a MOT retest but in the meantime have ordered a new lambda sensor and also advanced the timing a little which has made a massive difference to the responsiveness of the car.

Thanks for all the comments so far.


A further update. After replacing the manifold lamba and wiring, our eunos had another emissions test.

This time it passed on the lambda check, but failed on the CO emissions.

Requirement is 0.3% but we are hitting 0.45%. So I think it’s not now a lambda issue but something else.

I’m thinking of running some cat / fuel cleaner through the system to see if that’s helps. But otherwise interested to know if anyone else has had or solved this problem.


New cat fixed it for me. Aftermarket cats seemed to last about 2 years and scraped me through. Factory cat off a scrapper gave “perfect” numbers.

My car is a 1996 1.8 Eunos Roadster, so originally came with a 375mm cat. Given 96 imports had to conform to the cat test, I suspect the original factory set up was marginal at best; UK 1.8s had originally a 440mm cat (like the US) and later a 470mm cat (which the UK equivalent of mine would have had).

But, I changed the manifold to a 4-2-1 item from MX5parts, originally with a really badly made adapter piece from them that allowed the 375mm cat to be fitted (badly made, because it pushed the cat-back out of line, gouging the PPF). Eventually, I changed that to 440mm aftermarket.

Not sure you have a 1.6 or 1.8, as 1993 was the crossover year. In principle, you can decat your car, but I think if a cat is fitted, it has to go through the cat test. I’m not sure the 375mm cat off a 1.6 will fit the 1.8.

Non-cat test is 3.5% CO.

Try a cat cleaner, as you have nothing to lose.

Thanks AST. Mine is a 93 1.6. It’s not passing the decat test even though it has a cat. Currently at 4.9% CO. Cat and fuel cleaner going in today and will likely fit a new air filter also.

Will keep the forum posted if results.