P0420 Honda Code - Catalyst System Efficiency Below Threshold

I have a 2003 Honda Civic EX 1.7 V- tech.
Code P0420 comes up when the check engine light comes on.
My temperature gauge was dropping to cold when the car has been running for awhile, and the code P0420 would come up.
I then replaced the coolant temperature sensor, now my gauge works properly, and the PCM is getting better information.
But the code still comes up. No other codes come up.
Do you think it is the catalytic converter?

Yes I have that problem in my car 98 honda civic, I change the exhaust sistema complet for aftermarket and start give me the check engine light. Change you catalytic converter and for sure that gonna be you problem.

I have a 2012 accord lx 4cyl with 170K miles car has been great but the end light came on code po420 and autozo me told me it needs converter but the car runs great

Yes, for the P0420 code the problem is usually the catalytic converter. Most of the times a bad catalytic converter won't cause any engine performance problems, it will only affect the engine emissions, and your vehicle won't pass the smog check.

I have the same car time and the same problem i change the sensor oxígeno num 2 becas e i have the Conde po130 or 138 i think but at week the light back and i reset but now is another Code po420 and the mecanic say is the catalyst but is to expensive 400 hundred i try to change the sensor #1 i hope so work..

my disgnosis is reading Catalyst system efficiency below threshold (band 1). the car has be jerking once in a while since then. What do I need to change. Someone told me it is O2 sensor?

Sometimes is the O2 sensor, but from my experience most of the time is the Catalytic Converter.

I changed the catalytic converter and got the code again after 300 miles. Reason?

Did you used a new factory catalytic converter or aftermarket?

I have an Accord 2010 and it appeared a while back then disappeared. Now it showed up again and I got it checked up at an autozone. I don't know if I should be driving with the current problem since I am a college student who travels to class and work almost everyday. I will be going to the dealership soon.

Yes you can drive it, the problem is that it wont pass the smog check. Try using high octane gas, sometimes low octane gas could trigger the P0420 code.

Thank you so much for the helpful information. I'll be sure to try that soon.

i have the same problem.. i have changed O2 sensor one of them .. but the error came back again . should i change the other 02 sensor or not

What model and year is your Honda? Although some times the O2 sensors will fix the P0420 code, most of the times the problem is the Catalytic Converter itself.

So I get this code as well. But it is intermittent. Sometimes if the check engine light is one for a few days it will turn off itself. I would assume if the Catalytic converter was bad, the light would consistently stay on? Any thoughts to this?

Try using high octane gas for a couple of weeks to see if that makes any difference

I have a 2003 Honda Civic lx 135xxx miles and I keep getting the p0420 code. I have a new cat, maybe a year old, new spark plugs and coils. After about 6 months the code came back. Idk if i just got a bad cat or could there be something else. Did a little research and said there may be an air leak somewhere, but can’t seem to find it.

Did you use a factory cat? aftermarket cats tend to fail prematurely. Also, bad gas was probably the reason the cat failed the first time and if you din't change the gas type and/or station you may have damage the new one as well. Injector leaks, exhaust leaks, bad O2 sensors may also cause a prematurely failure of the cat.

I have question for you,I put new flex pipe 2inch new and universal catalyst 2 new oxigen sensor and my check engine still on,fail emissions,what you can recommend to do,I need help,Jonda Accord 2003 lx manual

You could try reprogramming the Engine Control Module (ECM), but sometimes universal sensors and catalytic converter do not meet the factory specification and may not fix the problem.

I have this check engine code too but which catalyic converter or o2 sensor do I change? The 2012 Honda Odyssey has 3 of each. 3 Converters and 3 o2 Sensors.

The P0420 is for the Catalytic on the back of the engine (Also know as "Bank 1 Catalytic Converter" or "Rear Catalytic Converter"). Some people claim that replacing the Oxygen (O2) sensors will fix the P0420 code, but I never seen that, usually the problem is the Catalytic Converter itself.

02 sensor circuit high voltage bank 1 sensor 1

I am traveling, and I went from my room in st. George, Utah (2000 feet elevation) to the north rim of the grand canyon (8800 feet) and on the way back the check engine light comes on. I check the code and its the p0420. would the rapid change in altitude (air getting thinner) have anything to do with it?

Yes, change on altitude can affect the Oxygen Sensor reading and that could set the P0420 code.

My car is a 2007 Honda Civic I got a code po420 I changed the up stream but my scanner gave me also p0340 also p01157 I went to clear before I change the down stream and the p01157 would not clear it keeps coming back does that mean the down stream please answer thank you

The P1157 code is for the Air/Fuel Ratio Sensor (Front Oxygen Sensor)

P1157 Honda Code - Air/Fuel Ratio Sensor 1 Range/Performance Problem
Read more: https://www.autocodes.com/p...

There is a video on the code page with instruction on how to replace the sensor