During the diagnosis of my 99 Subaru Forester, I frequently reset the ECU to clear P0420 and sometimes the code would not return for long periods (months or more), but would inevitably return and finally return became more frequent (week or less). Post-cat O2 sensor voltage pattern looked like the pre-cat O2 sensor, frequent voltage changes (1-2/sec) from 0.2-0.7 volts. Sometimes the pattern may be interrupted for a short time, but would return.

I used my Harrison Eng. OBD2 scanner (Obdscan) to monitor O2 sensors voltages. It's software, and hardware computer interface that allows you to use a portable PC to monitor your OBD2 system. It cost me about $200 for everything including a used portable IBM laptop. This tool has proved valuable in running down DTC's from my OBD2 vehicles. There are other brands of scanners readily available. I chose this one because of reputation for compatibility w/ all OBD2 systems (they're supposed to be universally compatible but in fact are not).

good luck

 

Texas and others...
I am a recent recipient of a P0420.
2007, 2.4, 5MT, 173,000 miles.

All was well with the car but a periodic code for slow response on O2 ( it I remember it might have been P0139,,not sure, but do recall it was exhaust sensor.

Car runs great....What changed. I just put a K and N typhoon intake on. 2 days ago. No other mods. Still running well.

The code came up within a day.

I reset the code once already and it popped up quick within 10 miles after reset.

I have changed a cat on a 99 acura CL 2.3 before and didn't have fun.
Your thoughts are appreciated.

 

I've heard of others having problems w/ CAI's causing P0420. This was on Subaru Foresters, not Accords. Thankfully, this code is much rarer for Accords.

Easy fix is to re-install OEM intake system. Hopefully you hung on to those parts?

good luck

 

I have this p0420 code on my 96' Honda accord. I'm getting 23 MPG driving the same route.

It's been on there since October. I reset it and it comes back 2-5 days later. Just recently I read about sea foam.

I put 3 ounces in my throttle body vacuum today, smoked a fair bit, put half in the tank and the rest down the exhaust manifold through the 02 sensor port and it smoked A LOT. I couldn't see the car behind me. People going down the highway probably thought my car was on fire.

I bought another can of sea foam and I'm going to put the full 12 oz in it tomorrow. Maybe if I'm lucky it will clear the code. If not for $5 it's worth seeing the look on people's faces!

Short of that, I'll take the cat off and manually soak it in some cleaner. Short of rusting from the inside out, I don't think there's any reason a catalytic converter can't last 20-30 years at least. From what I read cats break down mechanically, loses catalyst and lose surface area to control emissions. As mine doesn't rattle it's still mechanically fine and there's a chance a cleaning might save it.

I'll keep you guys posted.

 

See Scotty Kilmer's piece on youtube about cleaning catalytic converter. Worth a try as this part is easy to access on Accords.

good luck

 

I think his advice about cleaning the cat manually is harmless. The worst thing that would happen is one waste's their time...

I'm still hoping the sea foam down the exhaust manifold cures my cat problems. I put in that full 12 oz today and I drove to work 14 miles, I drove back, hit an incline and it was still smoking. From what I understand of sea foam

Although tomorrow I'll be replacing my spark plugs and checking my EGR valve. I hope it isn't the EGR system that's in my engine block, that would be a fair bit of work.

 

Did the defouler trick on my 2000 honda and the CEL is gone! I believe I have vastly improved and simplified the famous defouler trick. No drilling. Being a welder, I went to Advance Auto and bought a pair of defoulers in the help section and took them to my shop. Drilling etc. makes the remaining threads too thin and they break off when screwing into the CAT. I simply sawed the threads off one defouler and welded it onto the other one. Secondary o2 sensor screwed into this perfectly and then into the Cat. CEL light goes out just like when car was new. Awesome. 5 bucks. Contact me if you need help with this. Steve

 

I tightened up the bolts on my exhaust manifold and mines yet to come back. It's day five and it usually comes back by now.(Fingers crossed) In all previous clearings of the code it's come back within a 2-5 day period very consistently.

I checked the voltage readings on my 02 sensor with a code reader and sensor the front one is all over the place from .012, .0125, all the way up to .7 volts, whereas the cat 02 sensor ranges from .7 to .75 to .825, much more steady. The last sensor I had followed the front 02 sensor up and down all over the place!

Does anybody know if my Honda has a splash-guard for the cat? Mines rusty as all get out, and if I could find some kind of splash guard, it may just prolong it's life.

 

Your reading on the O2 sensors sound about right. The pre-cat O2 sensor will have a range between 0.1V and 1.0V. The one after the catalytic converter will say steady.

For the heat shield, you would have to get this from the dealership or an online store that sells OEM parts.