P0420 code repair?

You could either have a bad O2 sensor on that bank, or the catalytic converter needs replaced. You need a live scanner that can read the voltage from the O2 sensor to confirm it is working correctly. From that reading, you can determine which to replace. NOTE: The O2 sensor can damage the converter if it is running too rich.

I'm starting to get the p0420 trouble code in my expedition. I had it in a shop awhile back and the mechanic put in a "bypass" now the error code has come back . Does this mean that the catalytic is going bad or not??

code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions Some suggested steps for troubleshooting a P0420 code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter