After a week of owning my 2006 Xj6 it became apparent that all was not well. Although the car drove fine i was averaging 14mpg with my LTFT & STFT at 20 and 33.

Here's what i did to solve things

1, Ran 2 bottles of Cataclean through 40 litres of fuel which immediately sorted my P0420.

For the P0171 & P0420 ( lean on both banks) i replaced the following parts with new items:

Maf sensor (dirty)
Air filter (to short for the housing)
Spark plugs (service requirement)
Fuel filter (long overdue service requirement)
PCV valve and Hose (faulty)
Brake booster and under throttle body quick release O rings (faulty)
Upper inlet manifold seals (just for good measure)
Both IMT seals (just for good measure)

I also checked the one way valve on the small vac line fed from the rear of the manifold.
Repaired a broken poor previous repair to the evap control valve solenoid and while doing noticed that the vac line from the fuel pressure regulator on the injector rail
was disconnected.

I also found the inlet hose clip loose!

While i was in the area i also cleaned up the earth contacts behind the headlamps (and only had to drill one hole for the stud that fell off).
Hard reset (battery leads off)

No more codes and now averaging 26mpg and rising not to mention a hell of a lot more power.
I hope my findings help others faced with these common codes.

Finally, many thanks to those that have contributed on this forum regarding such faults and helping me familiarize myself with the AJV6 in such a short time.

 

Just a quick update. After carrying out the work above I was still getting the occasional PO420 and P0171.
Things were beginning to look like I would have to replace cats and 02 sensors, however a post from Don b regarding air flow meters
prompted me to clean the old air flow meter and retro fit it. No problems/codes now and hasn't been for several thousand miles!
It turns out that the new denso meter I bought was not compatible for my car despite its ebay listing.
What really threw me was the codes that were being generated was showing a problem with only one bank and not both as I would expect when dealing with an air flow meter problem.

 

You've done really well fixing that car !!

 

Thank you, even though I have considerable trade experience this forum has been an invaluable source of help.

 

Cataclean worked on my 1997 XJ6, 114,000 miles. Barely passed the Nitrous Oxide limit though.