MOT Fail O2 1998 NA... SOLVED !

Hi All,

I have read numerous posts on this subject, and am going to test the lambda sensor this afternoon.

The question I am asking is that the emission readings have changed so drastically that I think I may have missed something whilst refurbing the NA over the last few months. Perhaps someone more knowledgeable can point me in the right direction.

The MOT test was carried out in France. 

I have had the car tested twice in the last year and only driven 234 miles between tests due to the renovation work.

As part of the renovation I have changed the oil, air filter, fuel filter and plugs. The timing belt was also changed. The car runs sweetly.

I have removed the engine and box during the work. I have also sprayed the downpipe which houses the lambda sensor, although the sensor was removed during this process. It was stored safely, but perhaps was damaged during removal? Are they that fragile? I have ensured that the O2 sensor is connected 

The readings from the last test were

O2 Idle 0.32% ( < 0.5% )

O2 fast idle 0.3% ( < or = 0.3% )

Lambda 1.030 ( 0.97 to 1.03 )

234 miles later

O2 idle 4.22%

O2 fast idle 4.56%

Lambda 0.88

The MOT technicien indicated he thought it may be the lambda sensor. I asked why as that was the only reding which had not only passed, but improved. He stated that if the O2 readings changed, so would the lambda reading. In his experience the lambda was the issue, not the cat.

This is an area out of my field of experience, so any and all help greatly appreciated.

Hi Richard, why do you say the Lamda has passed? The spec is 0.97 to 1.03 and your reading was 0.88 so below the minimum. Sorry if you already know this but properly testing a Lamda sensor in the car is tricky. You need an oscilloscope as the output, if working correctly, swings between 0 and 1 volt about 8 to 10 times a second and a meter or DVM won’t show this.
Good luck.

A Lambda of 0.88 isn’t going to pass the MoT and indicates a rich fuel/air mixture. You can check for sensor error codes and the operation of the sensor, there are instructions in the technical area of the forum. Bear in mind that you may get a 4 digit error code as you have a late NA.

Hi all

Thanks for your speedy replies as ever 

I think I have found the ( MY ! ) problem.

I went to remove the lambda sensor to bench test it and it was loose in the downpipe. I could see a couple of the threads on the sensor and removed it by hand. I guess I didn,t nip it up after reinstalling it. Durrrrrrrrr…

I bench tested it, reinstalled it, tested in situ and achieved the figures indicated in this thread.

I had to rev the engine up and down to achieve the less than 0.2v and more than 0.7v, but I guess using a multimeter rather than an oscilloscope, that’s the best I can hope for.

Dave, my mistake, I didn’t read the figures properly regarding the lambda sensor. I was so gutted that the car had failed on just its emissions after a full stripdown and rebuild. I have stripped the shell completely, subframes and wishbones refurbed, new bushes all round, painted / rust protected everything, replaced shockers, brake lines replaced, brakes, calipers, clutch, cambelt and water pump, countless other things, interior from a V Special, new hood and frame from an NBFL, refurbed the wheels, new tyres all round, hunter 4 wheel alignment, and wanted to get a fresh MOT with no faults and no advisories…if it turns out that the only fault is with me not tightening the lambda sensor up I will be so annoyed with myself…on the other hand…for a DIY enthusiast…it will be a great result, and hopefully I will have finished spending…except for a new interior from seat surgeons…and a fresh paint job !

I will post the results of the retest.

If anyone thinks the above may not solve the problem, please let me know as I would hate to waste the retest fee lol


85% lambda sensor not screwed in properly.

10% you have damaged the cat due to the overich mixture due to the loose lambda.

5% you have done something silly in the engine bay you have not noticed yet.

I Hope your percentage guess is spot on. Re test booked for thursday afternoon…time will tell. Thanks Richard

One other thing that can cause an over-rich mixture is a failed inlet air temperature sensor. If it gives an incorrect reading, the ECU will give extra fuel.

This happened on my other car (not an MX-5) when a backfire into the inlet plenum physically burned the sensor.

Your car looks stunning. I know it’s been a long journey for you but hopefully the end is now clearly in view.
Let us know how it goes Thursday.
Good luck.

An Happy update to this thread !

Went for the retest today after tightening the lambda sensor…and passed !

Today’s readings were O2 idle 0.41         O2 Fast idle 0.19          Lambda 1.01

Such a stupid mistake to make…cost me €25 for the retest…but could have been much more expensive.

Thanks to all who contributed to this thread.

Happy Days 


Good News!