Golf -04/ "Emissions workshop" / help pls

[missfinland]

I'm driving a Golf -04 with 65K.
The "Emissions workshop" light went on on my way to work so went via VW and they fixed it saying they updated the software. The next day the light went on again and this time there was a screwnut or something similar that needed to be replcaed. The light was neutralized during the check up and before the sparepart had arrived the light went on for the 3rd time!! I got it back from VW yesterday and after driving for 50 km's it f**king went on for the 4th time!!
Otherwise works more than well, some momentary vibration can be felt in the steeringwheel and when put to move the consumptionmeter often jumps up to 99,9 l./ 100km. Is this normal? Can this have something to do with the light ?
I know that this emissions workshop light reacts to anykind of faultmessages that is enginerelated but the constant visits at the VW shop/service is annoying, time-taking and eats up a lot of money. So I'm writing here hoping to get a better understanding of the problem and maybe some hints of what signs indicates what kind of problem. Or even better... If some miracle-man/woman can tell me where the problem is Thanx, -Eve

[veedubmechanic]

without knowing the codes in the ecm we cant do much. It should say on the paperwork from the dealer what codes were in the system that caused the light.

[missfinland]

Ok, I will get back to you as soon as I have the ECM codes. Thanx, -Eve

[missfinland]

A call to the service and I found out that the code was PO171. Maybe heating element-related ? They said they changed a pipe, previously they were talking about a nut..
And last Saturday just out of nothing the lamp went off, was off for about a week until it suddenly today lit again when I was setting the cruiser..

[veedubmechanic]

thats a system too lean code. Usually caused by the Mass air flow sensor. If its a 2.0l a common issue is the fuel pressure regulator vacuum hose. Its under the engine cover on the passenger side and the hose usually cracks and comes loose. Thats usually about the only normal cause of the code other than a bad maf sensor.