Catalysist Efficency ... P0420 Code ????

What causes this code? It is listed as Bank 1


CATALYSIST EFFICENCY BELOW THRESHOLD BANK 1

Can this be a bad Oxyge Sensor?

This means that the oxygen sensor downstream (after) the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs).

possible causes are;
- An oxygen sensor is not reading (functioning) properly
- The engine coolant temperature sensor is not working properly
- Damaged exhaust manifold / catalytic converter / exhaust pipe
- Retarded spark timing

Best thing to do is reset the code and see if it comes back.
If it does then test both of the O2 sensors.

- measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter).
- The voltage should be low and steady (~200mV, depends on the vehicle).
- If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter.

Also many vehicle manufacturers offer a longer warranty on emissions-related parts.
So if you have a newer car, but it's out of it's bumper-to-bumper warranty, there still may be warranty on your Cat.

And GM might have extended that original warranty on your model.

Hope that helps!

More Specifics regarding EP 0420 Code. The check engine light came on after the car was restarted after a long drive, approx 100 miles.
Cleared the code and the light again came on after restarting after a 250 mile drive.
It comes on Immediately upon startup after the long drives.
Convertor or sensors?
Sensors are expensive so not willing to throw good after bad. For the next three weeks, the car will be 400 miles away.
Only has 72,000 miles on it. Just changed fuel pump and battery and fuel filter. Filter had indications of dirty gas in the past...black residue in filter and on pick-up screen in tank. Could that have fouled the convertor?
Thanks

If it is your CAT and this is on your 2003 GA ,
thier is an extended warranty on that CAT.

Though I would think its the O2 sensor bad.
Test the O2 sensors to find out if they are bad.

and the black residue that came out of the gas tank,
that is normal.