Can anyone help plz 2 weeks ago my engine management light came on so I had it scanned and the error code was p0420 efficiency level below threshold on bank 1 I have put a brand new o2 sensor in as I was told it was the fault with the sensor at the top of engine now this morning engine management light is back on ? I dont know where to go from here 

Bank 1 could be either sensor as its reporting the cat as being faulty. It's usually triggered by the 2 o2 sensors reading too similar.

Is it the same code coming up now you've changed the sensor?

It would be handy to get some live readings too.

Am  currently at work it came on when I was on way to work I can get it read later on today 

A have got my car scanned again and still showing same message efficiency level below threshold bank 1 the oxigne sensor is brand new what other could it be is there other sensors that could cause this message ? 

Bank 1 is a bit misleading on an inline 4 cylinder engine. It's meant for larger engines (such as a v8) which will have an exhaust manifold at both sides of the engine, and therefore double the amount of o2 sensors.

That code isn't referring to the sensor directly, rather the fact the cat isn't doing its job. Like I said before it would be nice to see some live data from the pre and post cat sensor.

I'd imagine either the post cat sensor has failed (or the wiring is damaged) or the cat itself is damaged and not doing its job properly. Hearing rattling noises from the exhaust, or the fumes smelling really bad are signs of a failing cat.

Thanks for your reply a will need to take it to my local garage on Tuesday i hope its not the cat as this would be a very expensive repair so i have heard 

I will let you know what happens on Tuesday is there anything I should mention to garage on Tuesday ie air intake as a have heard of people getting p0420 and it was a air intake gasket failure 

If you're just going to take it to a garage then drop it off, and let them deal with it. If you start making suggestions about how they should do their job, it'll just make them ignore you. After all, if you knew what the fix were, you wouldn't be spending money sending it to the garage...

Engine management light is back on this morning still reading the same code efficiency threshold below bank 1 but now says permanent ? I changed the spark plugs cleaned the mass sensor with some electrical cleaner but cant get the code to disappear is there a way to get this deleted I was told if I remove the positive cable from the battery that would erase the code at this point am willing to try anything 

Just plug your code reader in and press erase codes.

If its telling you the o2 sensor is faulty then I'd at least start by changing the other sensor? Or find out which one is faulty by looking at live data.

I have changed the o2 sensor but it wont let me erase the code saying permanent I will try disconnecting the battery and leave it for 10 minutes then connect it back then plug the Reader in see what its saying then 

More than likely your cat has failed, I have diagnosed / replaced quite a few on 1 litres. You will not be able to clear a permanent DTC as the engine management system has to see that that fault is fixed first by doing a drive cycle. 👍