Failed M.O.T emissions

[froggy10]

Hi guys .............. and gals. This is my first post

My Lag failed it's MOT emissions tests. Garage says it could be due to cat or O2 sensor or both. I don't really want to pay extra for a diagnostics test if I can get away with it. I have posted the results. If any of you guys could throw some light on the matter it would be much appreciated.

Idle
CO 0.11% PASS

Fast idle
CO 0.66% FAIL
HC 68ppm PASS
^ 1.001 PASS

2nd Fast idle
CO 0.56% FAIL
HC 59ppm PASS
^ 0.998 PASS

Thanks.

[roby37]

Well it ain't far out! Does it have 2 sensors, before and after the cat? Is the air filter clean, the engine oil level OK (not too high) and the oil not too old?
Probability is the upstream O2 sensor failing. It can be tested for voltage output which should switch between 0.2V and 0.8V, if producing 0.6V it is on the way out.
Ultimately if it is trial and error the O2 sensor is a lot cheaper than the cat and a new sensor would not be damaged by a faulty cat.

Add.
An O2 sensor would cost about the same as a diagnostics check.

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[froggy10]

Not sure if it as one or two 02 sensors. The oil was 60c when tested. I bought the car a year ago and it should have been serviced then, but I'm rather trusting and didn't bother to physically check. It has only done 5000 miles of mostly town driving since I've had it. Do you think a change of filter ,oil ,and plugs could get it past the emissions test. As you say, it is not that far out.

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[roby37]

"Do you think a change of filter ,oil ,and plugs could get it past the emissions test."
In all honesty no! You might check the plugs but if one/more was misfiring the CO and HC would be higher.
The air filter would be worth a look but it would need to be very dirty as a choked filter is, within limits, compensated for by the O2 sensor, if it is working properly. Removing the air filter completely for the test might help in a marginal case. The within spec CO at idle and not at fast idle rather suggests a possible dirty filter.
The most likely culprit is a sluggish(failing) O2 sensor, had it failed completely the CO would be much(6%-9%) higher.

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[light47]

wouldnt of thourght it was a problem with the cat you usually would be aware if the cat goes a bad eggy smell, but the plugs and air filter should help but still not a gurantee tough one to call really