High CO2 emissions

[manity]

Hi all could anyone help with an emissions question..

i have a 2004 1.4 polo it recently failed its MOT ( natural idle CO2 = 0.40 pass .... second idle CO2 = 0.42 fail and fast idle CO2 0.44 fail).

i changed the air filter and plugs added some injector cleaner and took it for a 20 mile run in 3 ( automatic). its wifes car and only does around town driving about 10 miles a day max so expected it was a bit clogged up.

as a result 2nd MOT reading were a close pass ( natural idle CO2 = 0.32 pass ... second idle CO2 = 0.30 pass and fast idle CO2 0.34 fail).

i have checked there are no fault codes and although it passed just. the readings still seem high ( what should they be? average).

ive included a screen shot of MOT results as a pic.

any help would be great thanks.

https://ibb.co/18g6vhc

[littlepolo]

Difficult to say what is the root cause of the emissions issues without knowing the history of the engine and its maintenance. At that mileage it could start to be problematic depending on its history. As an example, I swapped the head assembly on a 1.4 BBY around 90K miles for a head assembly that had covered only 25K miles. The previous years emissions marginal pass on the original engine would have failed when the recent MOT changes came in. Fitted with the replacement head assembly it passed easily (with VERY low figures). On disassembly the original head showed issues with valve gear and I suspect this affected gas flow hence emissions.

It sounds like the figures could be due to wear and tear and changing the filters, plugs, etc. will only help to an extent if there are underlying issues with the engine. It might be worth some further investigation if you want to keep the vehicle. If it is the top end your best bet would be a replacement cylinder head from a low mileage engine.