MoT Fail due to High CO % Volume Level

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: _NB
  2. I’m based near: __Manchester
  3. I’m looking for technical help or recommendations on: __ MOT Fail.

Hi All,
My 1999 NB has been laid up in the garage over winter 4.5 months approx.
Having conditioned the battery during the last couple of weeks yesterday I took it for its MOT.
It failed on CO% Vol which should be a max of 0.20 with mine reading 0.65!
Today I put some emission reducing Petrol adative in and took it for a 41 mile blast up and down the M60/M66.
It was then emission tested again this time showing 0.44 then 0.37 reducing to 0.25 before climbing up again.
Any thoughts?
Concensus seems to be to change oxygen sensor? If so which one genuine Mazda or IL Motorsport or other??.
Thanks
Cheers
Nic

2

Terraclean for about £100 will drop reading and improve engine response. Discount for MX-5 Owners Club members

3

Many Thanks I’ll make some enquiries tomorrow.
Nic

4

Give the additive more time to work and then give it a good hard thrash before next test.
I only use Forte additives, not sure what you used.
Obviously air filter must be good, well serviced etc.

5

I used Forte addative, how much time/miles does it take to take effect?
Thanks
Nic

6

I would think most of the tank in reality, can you post up the actual test readings?

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Was there also a Lambda reading as part of the test? What was the value? Should be close to 1.00.

High CO usually means too much fuel, so it might be an O2 sensor issue as ypu mentioned in the first post. The Lambda reading would help diagnose though.

Agreed that a proper thrash may help, using full revs at high throttle openings, rather than just motorway.

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The actual Lambda reading couldn’t be taken by my garage as they didn’t have the correct plug to read the engine management data from under the bonnet - they claim its an odd connector which wasn’t part of there kit!
Anyhow, I’ve now changed the plugs and fuel filter put some new fuel in and am off for another test😉

1 Like
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So 1.02 is the tailpipe reading? Things should be working ok in that respect then.

Good logic with fuel and plugs - enjoy the thrash! :grin:

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How they read the lambda reading for the MOT is something I hadn’t really thought of ‘till your post.
The MOT guys only put a sensor up the exhaust pipe so I ask myself how many samples a second can it do or over what time period does it measure lamda? Given a lambda sensor in normal operating conditions changes about 1 to 2 times a second I can’t believe this is detectable in the exhaust gas. This information can be read from a car with OBDII but never heard of an MOT station doing this.
Sorry just thinking aloud.
D

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Out loud thoughts are good! I’m not sure of the sampling resolution of your average gas analyser, but I’d have thought all of the emissions values are averaged out over however long the test period is.

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Thanks Guys for your thoughts.
Its wasn’t the MOT station who were doing the diagnostics!
Changed plugs and fuel filter, put new 97 octane fuel in and gave it a thrash!
Returned to MOT station and it failed again this time with higher CO% readings 55 & 44😥.
Next steps are Change Oxy sensor and Engine Coolant sensor on Monday and retest.
If it fails then its going on the basis of how unfair it is that hundreds of scroats are driving around with smoke pouring from their exhausts without tax mot or insurance!!!
Im now desperate as Im sure you can tell​:disappointed_relieved::disappointed_relieved::disappointed_relieved::disappointed_relieved:

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Ah man, sorry to hear that. Temp sensor is a reasonable shout too, if its out of spec it may be telling the ECU to stay in warmup cycle constantly and put more fuel in.

Should be able to test it with a mug of hot water, a multimeter, a thermometer, and the workshop manual for the reistance vs temperature specs.

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Lambda looks ok so and suggests the mixture is close to 14.7, i wonder if the cat is failing in this instance.

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Does anyone know whether changing the ignition timing can affect the co reading? I know some people do the 14 degree advance. My thought is that it would start the burn earlier and perhaps be a more complete burn by the time it exits the exhaust.

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If the lambda reading is OK but CO too high, I’d suspect a failing catalyst. Especially as it’s an NB.

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The Cat was replaced 5 years ago, since then the car has covered 10k approx so I wouldn’t expect it to fail at this stage.

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Hi Robert,
Given your response do you believe its still worthwhile replacing the Lambda and Temperature sensors given the cat is relatively new with low miles?

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I wouldn’t suspect the temp sensor at all. I’d test the lambda sensor before replacing it. It should output a rising and falling voltage between 0 volts and 1 volt at idle and a steady 2000 rpm. This might be hard to see on a digital multimeter though, try an analogue one if you have one. Test at the wires with the gold plated terminals.

Edit: Thinking about it, you may be able to test at the diagnostic plug under the bonnet from the MEN terminal. I’ll be able to confirm later when I’ve finished gardening.

1 Like
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Hi Robert,
Thanks for the reply - I don’t know how to use the under bonnet diagnostic terminal!!!
Thanks
Nic