My 2004 Volkswagon Polo has just failed its emissions test. It has failed on the CO only, everything else is okay. The results are as follows:

Engine/Oil Temperature: 92℃

(720 RPM) Low Idle
CO 0.32 vol% (Limit 0.30%)
HC 80 ppm

(2920 RPM) High Idle
Lambda: 1.02
CO 0.35 vol% (Limit 0.20%)
HC 76 ppm

The car has also been losing oil, I suspect at high RPMs. Can anyone give me some advice on where I should investigate?


Given what I know about the Polo, my first port of call would be to check the condition and operation of the crankcase ventilation. On the rear of the engine block is a breather "pod", a plastic part which has a hose coming up to the inlet manifold. I'd remove this and clean it thoroughly as they become blocked over time.

The other thing I'd check would be the vacuum takeoff hose for the brake servo. These become brittle and crack over time so eventually reach the point where they suck air in. This becomes a source of inducted air that isn't subject to being measured by the MAF sensor.

The other thing you may want to try is an "Italian Tuneup". Take the car on a long run at high speed to get the exhaust nice and hot. Particles can block the fine mesh in the catalytic converter reducing it's effectiveness.

Finally, before spending any money, you want to scan the car with VCDS or similar. Look for any indications of faults. I had a similar emissions reading on a 1997 Golf which was traced to a loose wire in the loom to the oxygen sensor in the exhaust pipe. Reconnecting the wire dropped the emissions readout by a magnitude of 10.

If none of this works, perform a full service focusing on changing the oil and filter, air filter, spark plus, distributor cap, rotor arm and possibly HT leads. Obviously do the fuel filter and cabin filter whilst you are there, just for completeness.

Good luck.

  • 1
    That is very helpful thank you! You were correct the vacuum hose was indeed cracked, going to replace that and check again.
    – colmlg
    Jan 11 '19 at 10:57

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