Help with P0420 code
 

I just got the P0420 code meaning I need a new catalytic converter and/or O2 sensor. What should I replace first? The o2 sensors or cat? I was wondering if I should go with the aftermarket cat or OEM cat? Also where do you guys recommend I buy the parts from?
Thanks!

OEM cat or high flow? How many miles on the car?

Aftermarket High Flow cats will eventually trigger the code you are experiencing...

If you indeed have the original cat on the car, here would be the order I would proceed in:

1) Clear code from ECU with an OBDII scanner and see if it returns... if you can put a few thousand miles on the car without the code returning, it was likely just a hicup in the ECU (happens occasionally).

2) If you tried step 1 and code returns, I would then replace the secondary O2 sensor

3) If steps 1 & 2 dont work, then replace the cat. Aftermarket High Flow cats will give you some added power (especially on an 04-05), but get ready to live with the light coming on. The defouler trick does not seem to work all that well on the TSX. If you have an 04-05, you can get an OEM cat from an 06-08 which should free up a bit of power without the issues of triggering the light. If you have an 06-08, replace with OEM as it already flows pretty well...

How many miles on the vehicle? Emissions system warrantee is 8/80,000 I think.
Check your owner's manual to make sure.

Poor fuel quality is the most common cause of this DTC. Note that P0420 does NOT indicate a bad O2 sensor, that is a different code. P0420 is simply the system running below a threshold that the system monitors.

mate try add a fuel injector cleaner and fill up with high octane fuel and dissconnect the battery and try a spirtive drive id say that should sort it.

My 04 Tsx has 135,000 miles on it. I treated the engine with some seafoam and reset the code and so far it hasnt come back on.

a lot cheaper then replacing cat converter and sensors, well done

I have an 04 tsx and when I had 103k my cat was out.... which was 3 days after I bought the car. That was $800 down the drain... I bought it wholesale though from my work.

Just came back on again. Looks like I need to replace that cat. Is it necessary to replace the O2 sensors too? Im kinda on a tight budget here.

I would start by trying new O2 sensors, or at least a pair from a known-good TSX if possible. If they still produce a code, then replace the cat and leave the sensors alone.

I dont know where I could get my hands on some O2 sensors without buying them new and they are quite expensive also. Hmm?

Have you tried top-tier fuel (from a near-empty tank), 91-octane? If after a full tank of that, the light still comes on after being reset, then look into a fuel injector cleaner as someone mentioned above.
If that still causes the light to come on, then the P0420 code should be taken as "legit" and you need to action accordingly. If it's "catalyst below threshold" then replace the catalytic converter.

I have had it come on twice so it must be legit. Here is the print out I got from Autozone.
Catalyst system efficiency below threshold bank 1
Probable cause
1. A/F ratio concern
2. Air leak near H02S(heated oxygen sensor) Bank 1
3. Ignition system concern
4. Faulty Catalytic converter

You think I just need a new Cat? Where do you buy parts from? Easy DIY install?

Dude, listen to what the printout is saying...A/F ratio concern is very likely to be fuel related.

1) Find a top tier station (list above) and a bottle of Techron Fuel System Cleaner.

2) Run tank to as empty as you feel comfortable (preferably need at least 15 gallons to fill-up.

3) Toss the cleaner in and fill up with PREMIUM gas.

4) Run car until nearly empty again.

5) Repeat at least the fill-up (Top tier PREMIUM)...more techron if you like, won't hurt.

6) Run it nearly empty again and fill-up again (Top tier, PREMIUM).

7) Reset CEL.

8) CEL resets, then you look at the air leak issue (should be noticable to the ear if you can check the vehicle from below safely at idle)...the ignition issue...and then the cat. A faultly O2 sensor (rather than an air leak) should return a different code.

91 octane is that the highest you can get, if so that is were your problem is you need at least 95octane in these honda/acura engines i run 99octane all the time thats the highest in ireland, germany you can get 104octane.

Nigel, it depends what method of calculating the octane the country uses.

In the USA, 91 octane is the Honda/Acura recommendation for the TSX. The US uses the AKI (or (R+M)/2) method of figuring octane (as does Canada). Europe uses just the RON number.

The 91 AKI is equivalent to a 95 RON European fuel...the 93 octane available in some parts of the US is like 98 RON fuel.

Just put some techron in and a full tank of BP 93 octane. Reset the light. Ill let ya know what happens.

i had the same code on my '02 nissan sentra Se-R a week ago. After trying to fix it cheaply costed me a motor. Now i have an '06 TSX. My cat was clogged and broken to peices because of that it clogged my second cat. By the time i fixed the first cat and drove to the mechani to fix my second cat, there was to much back pressure and my piston rings failed. Then the car was allowing oil in the cylinders and it was getting burned. Take it to a mechanic get a cat and make sure there isnt anything else clogged further down the exhaust line. It might cost you a motor going the cheap route. hope this helps!

Jason, the issue is that this code is very common from poor fuel...

its been over 700 miles since I poured techron fuel cleaner in and filled up with 93 octane and still havent had the engine light come back on. How many more miles until I am in the clear?

I'd say that's "intermittent" and you found the problem. Now just avoid the cheap gas and use Techron about every oil change and it should help keep it away.

UPDATE

Driving to work this morning the light came back. I had put about 1300 miles on it before the light flashed back... :(

OK...what gas is in the car? If it's an off or unknown brand:

Run the tank low, refill with known good gas and reset...reset if it comes back before you've run the good tank out...then refill with good and see what happens...

If it stays off, you might have bad gas shopping habits...comes back on then try looking at the ignition for a possible fault...then it's cat time...

thanks duck. It went back off on my way home this evening...weird.

Nah, with a fault like that, if the performance is borderline you can certainly get that kind of issue. Try taking a look at your ignition wires when you have a little time, just in case.

Same issue
 

Hey guys ... same issue in my car check engine light came on then back off reading the p0420 error. I’m going to attempt to flush the system and use better gas and hope I don’t have to replace the cat or sensors ... are there any reccomendations for which injector cleaner to use ? I’ll be shopping at autozone today after work thanks !

DTC P0420 - Service Bulletin 12-036
 

I found a Service Bulletin (12-036).
The symptom: the MIL is on and DTC P0420 (TWC catalyst system efficiency below threshold) code is stored.
Possible cause: The flapper valve in the fuel tank unit may rub on the fuel filter. Under low fuel levels this may cause a lean fuel condition, wich may result in damage to the WU-TWC.
It is recommended to replace the Fuel Tank Unit Reservoir (16011-STK-A00) and the Warm Up Three Way Catalyst (18190-RWC-A00).
See attachment!

Every single year since i have had the 1997 Acura RL i have had the check engine light to come on showing the failure code p420 . Luckily, for me last year i was able to get the car diagnostic scanner to erase the code, then i bought a bottle of a gas additive called CRC guaranteed to pass emmission. I then drove about 20 miles. This did the job and my car passed.. These rl before 2006 often have this issue, but changing parts is not always the solution. I have wasted money changing parts in the past and still the code did not go away. Hope this info helps someone. if you want to buy your own car diagnostic tool to erase the code on Acura. please contact me.
The car diagnostic tool always shows if your car is ready to pass the emission test.

Hey all, sorry to bump this thread but I wanted to keep on topic (Help with P0420 code) since I received the CEL code on my '08 Base TL (w DC Sports CAI mod).

So I filled with Chevron w/ Tech. 91, and added fuel treatment (VP Racing 7 in 1 Fuel Treatment), and I noticed the fuel burned away so quick compared to normal fuel gauge level movement.

1. Is it normal for fuel to burn faster than usual when adding a fuel additive?
2. I've noticed my fuel burning a little faster than normal months ago (maybe it was just me), but now I'm getting P0420...could there be a correlation here with fuel and P0420?
Note: I'll be resetting the CEL code this evening.

Thanks in advanced!