So I've had "P0420 Catalyst System Efficiency Below Threshold (Bank 1)" popping up from time to time over the last year. No rhyme or reason as to when it comes on. Each time I clear it and may be months before it comes back or could be days.

A search suggest that lots of faults can cause this.

Air leak (be it in the exhaust or inlet )
Misfire resulting in excessive hydrocarbons overwhelmingthe catalyst(dodgy plugs, leads, coilpacks, or injector/compression issue).
Excessive oil burning (as above)
Faulty pre-cat sensor (upstream)
Faulty post-cat sensor (downstream)
Sports cat/aftermarket exhaust (a common bodge on these aftermarket systems is to relocate the post cat sensor out of the main exhaust stream).
Map issue.
Dead or ineffective catalyst (rare unless its a non-genuine aftermarket cat (i.e like React) or a sports cat).

The most common suggest seems to be faulty rear O2 sensor. My car is an '02 WRX and has covered nearly 155k. So it would come as a surprise if either (or both) O2 sensors need replacing by now or indeed if the CAT was on it's way out. I have a replacement set of headers, decat up-pipe and Miltek sportscat to fit in the coming weeks. I was planning on renewing the sensors at the same time. My question is: am I causing any lasting damage to my engine whilst the ECU is seeing this fault?

The car doesn't noticeably drive any differently, there doesn't appear to be any effect on mileage, it went straight through the MoT without any emissions issues etc. I'm tempted to replace the rear O2 sensor now but I'd rather wait until the new exhaust is fitted and remapped in one go (seeing as I will be addressing many of the potential causes at the same time).

 

most common is not remapped after downpipe has been changed, ie a sports cat or decat.

 

My car is completely stock exhaust-wise.