Failed MOT, No Fault codes

[able]

I have had my 6n2 (2000, 1.4 16v) polo for 3 years. It failed its mot this year on emissions of fast idle CO 0.76, lambda 1.036, second idle CO 0.75 and Idle CO 0.70, it passed (Only just) on HC. The CO was 10.2 before it had all the vacuum hoses replaced! It had a full engine service (oil, all filters, plugs, leads,) 300 miles ago, it’s done 80k and is driven 60 miles each day to work down a motorway. No fault codes, very good compression, exhaust is good and have tried some cat cleaner, lambda works and output voltages tests OK. Engine runs really well and smooth.

it looks like it only just failed on emission, but what have I missed?

[alexperkins]

Wiring on your lambda sensor might be damaged - check the one on the front of the manifold

Otherwise, the cat might be on its way out. Sadly they dont last forever.

[able]

Ok, found the problem. Because the lambda/O2 sensor was running at below just below its correct operating voltage its giving false readings. This was because the battery is running at just below 11 volts. New battery!! No, the alternator is delivering just fewer than 11 volts regardless of load etc. One replacement alternator later and all is very good now,

[alexperkins]

Glad you found the problem