Hi everyone, 1st off nice forum Im a big europeen car lover(not japonese sorry) and a car technician.

Im working on a 2001 protege ES 2.0 auto

Problem is check engine light is "on" retreived the code P0421

Things I did already in my diagnostic :
-Back pressure test,under 1 ok
-Exhaust free of leaks ok
-With scanner, in data upstream O2 fluctuates from 0.1v to 0.8v correctly but heater changes from "on" and "off" each 5 sec......isnt it supposed to be "on" to heat up the O2 faster then heater is completely off????
-Also with scanner in data downstream O2 stays at 0.44v for a long time then climbs up to 0.70v and stays there,barely moving......the heater is always off too
-removed the cat,nothing loose and I see the light threw it lol

Whats wrong? downstream O2? cat? both?
I never guessed for a diagnostic and never will, I just want suggestions and tips, other ideas for tests....

Many people have this issue and most of them changed the 2 O2 and the cat .....I dont like the sound of that.....

THx for your help !!!!!!! Greetings from Montreal

This only code re appears every 4 days (when car is "hot" like a 50mph popping code, never when cold