I have a 2002 Eclipse GT, 56k miles, I'm getting a p0431 which is "Warm Up Catalyst Efficiency Below Threshold (Bank 2)". I get some sputter when at a full stop. Also loss of power accelerating. I replaced the after cat O2 sensor with no change, still get the code. Does anyone have any ideas?

More likely symptomatic of a problem with the precat on bank 2, my car threw this code and I reset the CEL hoping that it was just a faulty O2 sensor, but I don't have any of those problems. Might want to check for a vacuum leak also.

warm up catalyst on bank 2, translates to a problem in your #2 cylinder, it may also be the closest cylinder to your catalytic converter.

i had a similar check engine light code that posted almost the same thing. the problem shooting solution was that i test the pressure in bank 1 or cylinder number 1, i check for spark on number one cylinder, or i check my fluid levels. come to find out my wife had not checked the oil in 3 months and since i was never driving the car i never thought to do it. i was a full quart of oil low. i filled it up and reset the code. i hasn't come back yet. that was 2 weeks ago.

i would check your engine oil level. if it is low i suggest you top it off then reset the code. drive 5 miles, if it doesn't come back on then that is your engine oil level too low signal. if it does come back on then i would test the pressure in your cylinders man. you might have a faulty piston ring somewhere bringing your HP down and your Hydrocarbons in your exhaust up.

if its neither of those then test your spark plugs you may have a bad plug. i hope that helps you out bra happy hunting dude!!!!!

change plugs coils and wires, fixed my same problem

My car has run perfect until I replaced the battery. Now my idle is 600 - 400 rpm at times. Cold starts are good and the first 10 min seem normal, but when warm I have idle issue on my GT Eclipse. Idle Sensor? Reset Idle? Any other ideas?