I'll throw in my two penn'orth here as although it was a different vehicle, most of it is relevant.
Took my Rover 827 coupé for MoT on friday and all was going well until it went on the emissions test. Started off OK then all of a sudden the CO started climbing. Bringing the revs up made it climb even faster!
The PGMF-i ECU on that is equipped with two LEDs, one yellow and the other red. The yellow one comes on either flashing or steady for too high or low an idle speed respectively and the red one flashes a fault code.
At this point the yeloow LED was on steady but if the throttle was blipped, it would stay off until the revs were brought up again and returned to idle.
Knowing the cat has been "sensitive" in the past to getting cold, i took it for a blat up the road and back. This time the results were even worse! Tried a couple of obvious things but no joy so looked big and paid up and drove home.
Two miles from home, the PGMF-i warning light on the dash appeared. Looking under the seat at the ECU, the yellow LED was on and the red LED was giving one solitary flash every few seconds. This signifies the front bank (it's a V6) Lambda sensor so at least the fault was found.
Past experience has taught me to replace both so i've ordered a pair via ebay and popped the old ones out yesterday. The rear bank sensor was greyish-white and the front bank sensor greyish-black.
These are single wire sensors (no heaters) and are cheap to replace (got the pair delivered for under £25) unlike the Jeep sensors.
However the point of all this waffle holds true - they can (and will!) go at any time and usually give a few symptoms first.
I don't know if it's possible to swap the pre and post cat sensors over (only have the one sensor on my XJ) but if the heater is failing/failed in the post cat sensor then it may not be reading correctly. Swapping with the pre-cat sensor will allow more of the heat from the manifold to heat the sensor up and possibly get round the problem. If not the fault may well move with the sensor.
This is assuming that the plugs are the same on both sensors and that they are to all intents and purposes the same, just in different places.
Of course if the fault persists then it could be a faulty cat (blocked) and if it's only used off-road and doesn't need to comply with roadworthiness regulations, you could remove the cat and knock the innards out.
