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Yesterday I took my 2008 VW Jetta (1.4 TSI 122ps with 101k km) for annual emissions test. The car passed the test but I noticed that an increment in HC from year to year, especially when idling (650rpm).

2016 Test results
idle: CO 0.0% HC 1.0ppm Lambda -
2500rpm: CO 0.0% HC 2.0 ppm Lambda 1.01

2017 -- I have lost the results...

2018 Test results

idle: CO 0.20% HC 50 ppm Lambda -
2500rpm: CO 0.01% 9 ppm Lambda 0,998

2019 (yesterday)
idle: CO 0.01% HC 95 ppm Lambda -
2500 rpm: CO 0.01% HC 9 ppm Lambda 1,03

During the latest service (May 2019) I noticed that my old mechanic had installed wrong spark plugs (Intended for the 160ps version, 0.8mm spark gap instead of 0.9mm if I remember the correct values) and I changed them with the recommended ones. The fuel filter and air filter are relative new with less than 25000 km.

Two years before I had left the car to the mechanic for check because it had some strange idling behavior, the idle rpm were kept at 1100 rpm sometimes but that was because of a faulty battery. I don't know if somehow he reprogrammed (remapped) the idling in the ECU. ( I don't know if it is even possible apart from adjusting the idle rpm)

Apart from that I don't feel any significant power loss or misfires during driving. I haven't check for error codes with OBD but I hadn't had any Check Engine Light ever.

What can cause the increased HC? Is it something to worry about?

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    The CAT may not be working as efficiently as it gets older – HandyHowie Sep 8 '19 at 12:18
  • Is there something that I can do to check it or I just leave it as it is for now? – Mike T Sep 8 '19 at 12:19
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    I wouldn't worry about it for now. – HandyHowie Sep 8 '19 at 14:01

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