I was getting OBDII DTCs 0171 and 0440. The 0440 code was always in the "pending" status. The solution was to replace the exhaust gas recirculation solenoid with one from a salvage yard. The EGR solenoid was essentially free from the salvage yard because it was purchased with a bunch of other stuff. On my 1996 Volkswagen Jetta, the EGR solenoid is on the passenger side and slightly below the air filter. It's cylindrical.
Besides the DTCs, another symptom of the EGR solenoid malfunction was a high-pitched squealing sound. Just by sound, this could have been mechanical. However, it did not vary with engine or road speed, so my mechanical adviser concluded that the sound was electronic. Yet another symptom was occasional loss of power at low speed or surges of power.
I got through the emissions test even with this problem because the DTCs don't become permanent for a while. The trick is to reset your codes, use a modern code reader to find out when all of your monitors are in the "ready" state, and then get the emissions test before the permanent code is thrown (as opposed to "pending"). At least, that works in Georgia. Stricter states might fail due to pending codes.
The EGR system sends small amounts of evaporated fuel into the engine to burn it. My solenoid was getting stuck and squealing. The fuel system would assume that fuel would be coming from the EGR system, so, when it wasn't, I would lose power. Then, when the valve (solenoid) suddenly opened, there would be a surge of power. The sound only happened when my foot was off the gas pedal. Note that the sound was impossible to trace, even with a stethoscope. I figured out what the likely culprit was based on all the factors I've mentioned and looking in the Bentley Manual for a culprit that fit all the factors.
OBDII = On-board diagnostics, second generation. It's emissions control in all cars from 1996 on.
DTC = Diagnostic Trouble Code
The 2003-era ODBII reader I bought could not tell when the emissions monitors were ready or read pending codes. As of late 2009, I believe that most of the readers can. I'm almost certain that all of the Actron brand readers can.
VW Golf Jetta A3, OBD2, high-pitched noise, P0171, P0440. ODB II. ODB 2