I have a 1996 Jetta GL with a 2

What scan tool are you using?

Which sensor did you replace (location)?

Did you verify wiring integrity?

Does the speedo work?

Does it seem to show accurate speed?

The Instrument cluster sends vehicle speed to ECM, a faulty cluster (even though speed is shown) can set p0501 codes.

Thomas

There are rare cases where the speedo drive gear slips. Causing lack of speedo function (intermittent).

If your speedo always works, then you need to monitor the speed signal at the ECM, the signal it receives from the Instrument Cluster.

You can do this with a Digital Volt meter connected to the correct wire at the ECM harness plug, or via a good scan tool such as VCDS if your OBDII reader can not display live engine sensor input data.

Monitor the voltage while the car is driven or jack-up & support the car and then let it freewheel in gear while up in the air.

Wiggle the wire / harness all the way to the Instrument cluster to see if there are any sensitive spots to vibration / movement.

Might not be easy to track down. If you are lucky, it will be in the raintray area where the ECM is.

Good luck!

Thomas

Sorry, forgot to elaborate on the wiggle test.

From ECM to Instrument Cluster. Wiring from the sensor to the Cluster is probably OK
since the speedo never cuts out.

You would need a labscope to be able to catch any glitches that occur in milli seconds
that your eye can not see. Depending on the Digital Multimeter, it too might not catch
a glitch if its refresh rate is too slow.

Thomas