Having had an oil change service two weeks ago (& having done 2,700 miles since last year a check confirmed plugs & air filter fine) it was a surprise when MOT'd to get a CO reading at 0.20% at 2500-3000 rpm, 0.07% on idle.


Should I worry that the 0.20% reading is at the max for an MOT pass? The engine runs sweetly, at under 65k miles, & uses no more than half a litre of oil per 2k miles. The exhaust seems good - half being original (12 years old now) & everything back from the cat being from a newer car whose owner fitted a sports system.


At five previous MOT tests, the CO reading has never been more than 0.08% so I'm tempted to just cross fingers & leave it until next year - but would welcome any advice on what may avoid a further problem!

It is not unknown for the O2 sensor threads to become corroded, oxidise, contaminated over time.

I'm led to believe that removing the O2 sensor(s) and thoroughly cleaning the threads with a wire brush, and if necessary chasing the mounting boss threads through with the appropriate sized tap (or new/stainless steel bolt of same size) can work wonders in reducing conductivity/voltage losses and bring measured CO values back to normal.

Of course if you have a faulty sensor or a genuine fueling problem then the game plan changes.

High CO is usually caused by a faulty sensor or a bad air filter. Mainly MAF, lambda, temperature and throttle position sensor are the usual suspects. However I would assume a fault in any of these would cause an error code and light up the injection light (not 100% sure, check the factory manual) and all but temperature sensor will cause other symptoms too if they fail, so my best guess would be the air filter. Also if the CO level is low on idle but high when reving that might indicate a restricted airflow.
It's quite cheap and easy to replace, so maybe try a new air filter first?

Also cleaning the lambda like s130 suggested might help, as a working lambda with a faulty mounting will not cause and error in the ecu but may affect the measurement. Good tip s130.

And no, I would not worry about it too much at this point.

Although now that I think of it the MAF is located just after the air filter, so a restricted air flow would result only in lack of performance, not errors in MAF measurements. Might be worth cleaning or replacing the filter anyway.