2001 Jetta 1.8T 172 engine code

You really need to run the Diagnostic programme to get the fault code in order to pinpoint the faulty part. See here for possible fault -
http://www.audizine.com/forum/showthread.php/104097-B6-A4-1-8T-Engine-Speed-Sensor-%28G28%29-Replacement

Have you had the O2 sensors checked ? Code 172 is for a rich signal from bank one sensor one and code 156 is for a problem with bank two sensor two which is after the catalytic converter.
One of the injectors could be flooding a cylinder, or you could have a faulty sensor.

the temperature sensor can cause a rich mixture at the wrong time by providing a false reading of cold engine

If upstream sensors are over 100,000 miles
on any of your vehicles change them

Leave the downsteam ones alone

The 172 is rich ,so maybe you have a
vacuum leak & lean condition & the computer
drove fuel trim rich & set the 172

All that can be Googled as well as any codes

this code is a rich code your car is burning too rich...meaning not enough air is mixing with the fuel...make sure you check all lines insuring that bank one doesnt have a clogged vacuum line etc..Bank one is where cyllllinder one is located or usually if your facing the bell housing while sitting in the car it will be the right side of the engine...But I would start with hoses,then go from there...

it means that your fuel is running too rich,and needs more oxygen for a cleaner burn. i think that your oxygen sesor may be faulty,but have autozone run the trouble codes for you,and see what exact part is bad.

you might want to check the intake for leaks-let the engine run-while running spray some brake cleaner around the intake-if the engine boggs down or dies that means your intake gasket is leaking and needs replaced-this is very common

P171 Lean to bank one
P172 Rich to bank one
P174 Lean to bank two
P175 Rich to bank two.

These are conflicting codes. I would suspect that you have a bad MAF sensor. take it out and try and clean it before you replace it. It could also be a bad O2 sensor. But unlikely that both sides went bad at the same time.

Most likely Mass Air Flow Sensor. Could be oxygen sensors also but the mass air flow is more common

look for vacuum leaks the last 4 codes are lean codes
primarily lookm at the pcv hose noturious for collapsing