Nanaof4, I agree 200% with Pedro’s last comment,
“If all above is OK (good working condition) replace the three way Catalytic converter.”
Been there Done that..exact symptoms were on my 2004 Quest and after many suggestions and banging my head onto the several walls, finally the dealer was able to pinpoint the problem with the Catlytic converter and the exhaust tube.
What actually had happened, at around 100K miles the upstream catalytic converter (1 of 3) started to shed its inner linings and lodging into the lower exhaust tube resulting restricted exhaust flow, and the end result… unable to drive uphills or at < 45 mph the most with temp gauge gone hiking, most pre-symptoms were noticed by my wife who is the main driver, and some of the symptoms included rough idle, rattling as you gas pedal from stops, clunking noises in the engine. And obviously the same CEL warning on the dash with same codes for the “Catalyst efficiency below threshold” along with P0420. Stealer quoted me for $2400++ for the(Cat and the exhaust tube $1860)+ $640 labor. Since there aren’t any after market subtitutes for this model at least at that time, I was lucky to have the final try myself by removing the exhaust tube and banged it with the hammer followed by mallet for about 1 1/2 hr and got out 1600 grams of precious metals withouT the precious price;), i left alone the cat as is and reconnected it and since put on 11,000 miles and knocking the wood. Instead of $2400 it cost me 4 HOURS + $44.29 worth of propane torches to heat the tube and plenty of elbow grease…and I can’t complain anymore, hope that this helps in your troubleshoot. Looking forward to hear back from ya, AND NO NEED TO WORRY ABOUT YOUR KNOCK OR O2 SENSORS, CAT IS YOUR MAIN PROBLEM. 2004 Quest SL