NB carbon monoxide MOT fail

[lobster940]

Hi - 1999 1.8 MX-5 NB here. Failed its MOT due to excess carbon monoxide emissions.

Should I be fairly certain this is down to a failed oxygen/lambda sensor?

If so - is there only one sensor I need to replace, or two?

[wildoliver]

On NBS it is usually the cat that has failed. But look at your lambda reading, if the lambda reading is ok it will likely be the cat. If it's out it then it's possibly the lambda.

Be warned. Often the cat has failed due to oil contamination, if your car is burning oil then that explains it. This problem affects 1.8 NB cars (not nbfl) due to the oil control rings closing up.

[lobster940]

Thanks. The cat was replaced last year (after the previous one was stolen).

Aftermarket cat fitted but I assume this would last longer than a year (with limited mileage)?

Edited by lobster940 on Wednesday 25th January 15:01

[lobster940]

First fail on CO, second fail on lambda.

If the O2 sensor is shot, would a light (EML or otherwise) show up on the dashboard of an NB of this age (S-reg, 1999), or is it too old?

^^ Engine capacity is 1,800 cc - tester's typo.

Edited by lobster940 on Wednesday 25th January 15:01

[TVRees]

Those results are not very consistent. Might be worth a recheck.
You should get an error code if there is a problem. Have you got an OBD tester?

[wildoliver]

Sorry been away, as above. Very inconsistent. Could be a failed lambda, less likely to be a failed cat as it did go through the second time. It could be a bad tester, a good tester can squeak a borderline car through, just like a bad one can fail it on emissions, but the lambda makes me think you've either got an exhaust leak or a lambda failing. I would look/listen for an exhaust leak first. Head to manifold, cracked manifold, egr pipe off manifold, manifold to downpipe or downpipe to mid pipe being my prime guesses. A bad gasket would be enough to do that.

No obd on an NB1, but you can flash the codes using an led. But I doubt you will get much.