Hello,
Have fault code PO420 Catalyst Eff below threshold Bank 1. Do you think its a good idea to change O2 sensors (Both) before I bring it in to a mech and pay big$$?
Hoping its not CONVERTER!! Anyone know where good website to buy these sensors? Have 110,000 miles.
THX for any info
First thing I'd try is to reset the CEL and see if it comes back. Mileage and age is about ballpark for when these sensors could get "lazy" and need to be replaced. If you have a good multimeter and right probes (could fabricate something pretty quickly yourself) - you could try backprobing the sensors and see what they are really doing.
www.1sttoyotaparts.com and www.rockauto.com are two sites that get good reviews for replacement parts at a reasonable cost. I would suggest sticking with the Denso sensors over the other sensors out there. Too often using something else, example Bosch O2 sensors, tends to yield unusual behavior in drivability, persistant CEL, or poorer fuel economy. Last time I checked - OEM Denso sensors (just plug and play - no cutting or splicing of wires) - were around $110 for the upstream (pre-cat) and around $90 for the downstream (post-cat) sensor. The fault code you have indicates that the downstream O2 sensor readings are inconsistent with what they should be reading (catalytic converter seems like it is not working). Note that in 90% of the cases, the sensor is lazy or bad - the catalytic converter usually ends up being fine.
THX FOR THAT INFO. Any idea as to where to unplug downstream connector? Does the disconnect end up in passenger floor side? Guess I need to pull up carpet? Does upstream end up in same location?
Thx again!!!!
Bikeman982
If the O2 sensor is anything like most Corolla sensors, it has a short lead and the connection is most likely under the car.
I don't think you should have to take up the floor.
If the O2 sensor is anything like most Corolla sensors, it has a short lead and the connection is most likely under the car.Just changed downstream sensor. Did have to bring up Passanger carpet a bit to get to connector on this 2003. still have ck eng light on. Is it common to still have light illuminated? Will it go out by itself or should i disconnect Battery and reconnect to reset fault?
I don't think you should have to take up the floor.
Thanks..
Did you reset the ECU after installing a new O2 sensor. I guess you would have to do that as it may not reset the whole thing automatically. You can disconnect the negative terminal of the battery for about 5 min and it should do the trick. If you don't want to disconnect the battery then pull the EFI fuse from the fuse box for about 5 min.
Keep posted if changing the O2 sensor got rid of P0420 code.
Ajay
Light has been out for week, anyone think its necessary to chg upstream sensor as well? What benefit is there in changing sensor if check engine is not showing faults, 03 has 108000 on it. Why did PO420 code show when it was problem with downstream sensor? I should have gotten light for faulty sensor as well?
Anyway looks like the 800 dollar cat problem was nothing but an 83 dollar sensor. I wonder what the dealer would have tried to sell me? A sensor, A Cat or Cat and sensor. 
Amazon was great with this part, fast and relatively cheap for sensor. How much do you think dealer would have charged for sensor chg?
thx for all help!!!!!!
Change the upstream O2 sensor only if you feel that fuel economy has dropped significantly from normal. Example, if you have been averaging about 35MPG mix driving over several months - then you suddenly see it dropping to 30MPG and continuing to decline - then I would suspect a bad O2 sensor. They do have a finite lifespan - generally between 60K-120K miles. Some last much longer - there are a few that I know have gone over 200K+ miles on the OEM sensors.
P0420 (catalyst below efficiency) is mostly caused by a bad downstream O2 sensor. The on-board computer has no real way to test the downstream O2 sensor, except to check if the heater element is working - there is no "bad downstream O2 sensor CEL". Only thing it has to compare to - is the waveform from the upstream O2 sensor. If the catalytic converter is working properly - then the waveform before the cat and after should look different in a certain way. If they don't,you get a P0420 CEL set. Problem now is to verify that the waveform before you go further - many times, the O2 sensor will become "lazy" or read incorrectly. This will also set the P0420 code, as it satisfied the condition for the waveform being "different".
Have to view the CELs as a starting point for diagnostics. Too many times - I see a mechanic plug in a scanner and say "CEL code is --- - that means part X is bad" - then the mechanic didn't do their job correctly, they still need to diagnose the problem properly. If the onboard computer told you exactly what was wrong, we wouldn't need any mechanics.
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