P0420= catalyst below efficiency level-not a sensor problem. 4 or 6 cyl. engine? will determine what cat is needed . Have dealer check VIN# for possible software updates prior to throwing parts at it (may just be software error)................kinda like the very last line of answer #2.....I've only done ....like a hundred of those updates .for exactly that reason....never mind ....
The Check Engine Light (CEL) is on, with a code P0420.
1. Verify that the fuel trims are good at idle and when driving the vehicle. It is possible to have the fuel trim at 20% and not set a fuel trim code and cause the catalytic converter to not function properly.
2. If the fuel trim is only off on one bank, look for a faulty Air/Fuel (A/F) Ratio sensor, a vacuum leak or a fuel delivery problem on the bank the fuel trim is off on.
3. If the fuel trim is good and within 10% plus or minus, drive the vehicle at a steady throttle, steady cruise, and monitor the rear O2 sensor operation. If the rear O2 sensor is switching rich to lean, the catalytic converter is not storing oxygen correctly. When the catalytic converter functions correctly, the rear O2 sensor should not switch. If the rear O2 sensor switches at a steady cruise while driving, the catalytic converters are failing.
4. Since this vehicle uses Air/Fuel (A/F) Ratio sensors instead of front O2 sensors, if the vehicle has high mileage, it is always wise to replace the front A/F Ratio sensors if the catalytic converters are faulty and need to be replaced. In some cases the A/F Ratio sensors can cause a false code P0420.
Tech Tips: Refer to Honda TSB #03-063 for Engine Control Module (ECM) reflash for a 2003 Accord for this code.
Ugh! It is the catalytic converter, my friend. That code I just recently got on my Chevrolet Aveo and that is like the "Kiss of Death" -- Blue Screen of Death on the computer. This converter is a very major part of the vehicle, and if it goes bad or fails completely, you will have a major problem with your exhaust system, and it will cost you BIG in the long run. The code that my mechanic found this morning was an indication to me to get it done quickly and without rushing it, like most mechanics like to do, especially at gas stations. The first gas station I went to was going to charge me close to $2,000 for to replace it entirely. I asked why so high, but they didn't tell me. So I shopped around and went to another gas station near where I used to live, and they were going to charge me close to that price ($1,800) but still that was too pricey.
The next place I went to was National Tire and Battery and also to Merchant Tire -- NTB was going to charge me about $1,200 and Merchant Tire was going to charge me close to $1,400 or so. Then I called Midas, and they did a FULL diagnostic check on the Check Engine light and found the same code P0420 and changed the catalytic converter for a tune of $830, and that included discount of $20.00 as well, with a guaranty of 3 years on the converter (so they can replace it for FREE if it goes out again).
I would suggest taking your car over to Midas and asking them to do a Full diagnostic test on the Check Engine light to see if they get the same code or not. It will cost you about $100 for the test but you can get a coupon to reduce the price a little bit by going onto Midas's website. Then they will tell you after that, what the code(s) will say, and how much it would cost you, depending on the make and model of your vehicle.
I wish you luck, but be careful where you take your car to. It could save you a lot of money in the long run. Right now I saved close to $1,200 because I took my car to Midas rather than going to a gas station. And this thing is right up Midas's alley anyway because it deals with the exhaust system and muffler in a way that they are famous for.
To compare an avevo(DAEWOO) to a Honda is an extreme stretch. Take your car to a true pro. I do not mean a big chain store. All they care about is VOLUME not quality!
Ask your friend to guarnty it will fix the problem. I have been doing this buisnes for over 40+ years.
AVEO (Daewoo) is the biggest piece of crap. A turd is a turd no matter how yow pollish it.