P0420 on a 2.0TS

[Citroënbender]

Sort of a "Bazz Question".

I've had this recurring at low RPM and mostly when the engine is up to temp.  Sometimes it can be banished for a week or two, sometimes barely at all. This is on the 157K car I just did belts, plugs and variator on.  Sensor is genuine Bosch (ex Alfa Workshop) 18 months old,  thermostat April 2015 and temp is stable, oil Penrite 10W-60 which it does burn a bit of, cats are definitely not blocked, MAF new genuine January 2015, coils all test good at 7.9K Ohms.

I'm presuming the sensor reading is merely reflecting an upstream condition.  Should I be looking for (say) a lazy tappet or exhaust blow at the head?

[bazzbazz]

I am assuming this is on a 147 TS and all items you mention are operating as they should.

A lot of the times it is not the O2 sensor itself but connections. For example on the 147 TS one of the O2 sensor connectors is mounted on the side of the head between the head and gearbox at the front. Sometimes the  heat affects the connections and they lose contact. I solve that problem by using a pair of Forceps and twisting each pin in the grey plug 15 degrees so that it makes a tighter fit in the female pin.

The other cause of this is you have a leak in the exhaust. Can be caused by the following -

1/ O2 Sensor loose in its exhaust fitting
2/ Crack in any of the welds - this is usually caused by the exhaust taking a hit from large speed bump ect

Look for a whitish/grey powder flash from any of the welds where the exhaust gases have been leaking. If there is a crack/hole/leak it can be welded or sealed with Exhaust Putty.

Start with those and see where you get.

[Citroënbender]

Thanks, I know the grey sensor plug you mention, just below the MAF and above/forward of the thermostat. Had to move it with the head harness, for the belt change.  I'll start with the "pin tweak trick". 

The exhaust collectors and precats don't show signs of leaking on a quick perv but I'll check again in due course. 

[bazzbazz]

Don't forget to do it for ALL the O2 Sensor plugs, do one, do all while your there.