2004 Jetta GLS 1.8T poor gas mileage

2004 VW Jetta GLS 1.8T Triptronic 67,000 miles

7-8 months ago mileage went from 28-30 mpg every tank to 22-24mpg.

No check engine light, no codes (last service at 60,000), performance is very good, no detectable difference. I regularly check tire pressure, 34-36 psig, and driving habits have not changed. Air filter is clean. Service at recommended schedule by VW dealer techs.

Winter fuel formulation?
Ethanol mix?
Dirty MAF, AIT?
Degraded O2 sensor?
Faulty CST?

If need be, is it relatively easy enought to get to the AIT? CST? O2 sensor(s)?

I love the car but would love to get back the better mileage.


thanks

Alan Fraser