With this new information, I would still be reluctant to replace the CAT. Everyone was telling me to replace my CAT, but once I replaced the parts on the EVAP system, everything was fine.


The reason for the P0420, has to do with the way the O2 sensors work. There is one on the exhaust manifold and another one on the CAT. They work in tandem and if something is off, it can trigger a P0420 (even if the CAT is OK). Too much fuel will trigger P0420. A bad O2 sensor on the CAT, will also trigger P0420. I am wondering if your EVAP canister is saturated with fuel. Do you "top off the tank" when you fill it up with gas? Does the "check engine" light ONLY come on if you are going over 45 MPH (or ONLY on the highway)? If it comes on at higher speeds, you need to check the timing of the distributor cap (If yours has one, mine is a 1998 Accord)
EVEN bad spark plugs can trigger a P0420 (bad spark does not burn fuel that well and lets excess fuel into the CAT). Bad fuel injectors can do that if the spray is not a fine mist.


The reason you need to be sure of what is triggering the P4020 code, is you can replace the CAT. BUT... if you have not fixed the original problem, the CAT will go bad again, then you are back to square 1. CATs practically never go bad by themselves. Something CAUSED the CAT to go bad (USUALLY bad fuel/air ratio).


Just for starters.. try the following (these are cheap and easy things to do and you have nothing to lose):


1) replace the air filter
2) replace the fuel filter
3) check all vacuum lines
4) pour a can of B-12 into the fuel tank ( this will clean the injectors and help remove carbon from the pistons and valves.
5) clean the air intake valve (also known as the butterfly valve) with B-12 (NOT while the engine is running). just remove the air intake duct and you have access. spray and clean with B-12 and wipe dry. Be sure to let it dry off completely, or you will have a rough start and it will take a few seconds to smooth out. but best to wait like 15 minutes to be sure it is dry. reconnect the intake air duct, then start engine.
6) CHECK the fuel pressure regulator. pull the vacuum hose off the regulator and check if you smell or see fuel. if you see or smell fuel, the rubber diaphragm is cracked and leaking fuel into the intake manifold. ( HENCE extra fuel getting into the firing chamber, and unburned fuel gets into the CAT...triggering the P0420).


That's about all I can think of to start off with. If none of these help, then check the timing of the ignition, change spark plugs, sparkplug wires, ignition coil, distributor cap, and rotor.


good luck!

 

Hi rockhoundrob,

sorry, it has taken me so long and as of this evening the problem still persists ie P0420 code) and I still dont believe it is a cat issue and I have less than 60 days to fix it since it failed inspection lastt week.

Last weekend, I replaced spark plugs to NGK PZFR5F-11 (had autolite NAPA plugs) and replaced the O2 sensor. The P420 came back this evening after 104 miles of driving. The car drives, shifts smoothly now (definitely better experience dont know which of the 2 helped)


In addition, the mechanic used a camera (gizmo) and checked the cat, the geometric patterns on cat were seen and with no substrate fallen off. The 2 rows looked quite good. I coudlnt tell about coated-substrate that may reduce the efficiency of a cat (is there a fix for it?).


I suspect I still have idle issues (completely warmed car idles at 1000RPM) and can this be a possible cause?

There are no other DTCs other than P0420. So far, CEL has come on twice on local roads (Cruising to a halt at stop-lights, about to hit an uphill at 30-35mph) and once on a highway (speed 65mph on a uphill).

1. I changed air filter (STP) around 6 months ago.
2. I cannot change the fuel filter since it is in the gas tank
3. Right after the canister vent valve was closed, we checked for exhaust vaccum leaks.
4. distributor, rotor and cap was changed March 2013. The car ran fine(great gas mileage 30mpg) until late July when I had the car serviced (throttle body/IACV clean). The mechanic poured a can of techron (july)
5. Right after this repair, P1457 showed up right away. The car was running very rich for the first few days(idle was 2500rpm and driving in D4 with foot removed from gas pedal was 2000rpm) - I may have driven the car for nearly 3 weeks when P0420 showed up! I attributed this to the first repair but had no conclusive data to prove it out.


I will hunt down fuel pressure regulator this weekend and report.

 

Two things...

1) Idle should be around 650-700RPM. The fact that the RPM is MUCH higher is an indication of a vacuum leak or wrong idle fuel setting. Check for vacuum leaks first. Possibly idle control valve or throttle position sensor problems.

2) P1457 is EVAP leak problem. This is indicative of vacuum leak in some part of the EVAP system (including a loose fuel cap) and excess fuel is getting into the vacuum line and into the intake manifold. This is normal route, but you may be getting too much leakage. Do you smell fuel when you get out of the car?

What kind of year and model is your Honda?

I just re-read your post... I would have to suspect something about the throttle body cleaning. Pull the air intake hose off and look at the throttle. Open the flap valve and look in there, does it look clean?

Also check the fuel pressure regulator. Pull the vacuum hose off the regulator and see if you smell or see fuel seeping out. if you do, then replace the fuel pressure regulator.

 

will do and report this weekend. thanks for taking the time to post

 

I just thought of something else, when you look at the air intake (when you remove the air duct) check that the throttle valve is closed. If it is slightly open, that is another thing that will cause your engine to have high RPM.

 

update

Took the car to the mechanic and checked the following:
1. MAP - 7.5 inches of mercury and increased as we increased the throttle and dropped back to 7.5 inches when throttle was closed.
2. The O1 sensor was between 0.1-0.9V and O2 sensor showed voltages between .1-0.9volts at idle speed (650 RPM). With engine revving at 2500RPM, the O1 voltages was in the range...oddly enough O2 sensor voltages was also going all over the place...based on what I find online, the O2 voltages are supposed to hold steady between 0.1-0.5V
3. Really odd, the idle of the car was 650 RPM (this was after me driving a good 30 minutes to the mechanics shop). After a cold start, I would say approximately 10 minutes of driving and say 7-8 miles from home, I have seen the idle @ approx 1000rpm.
4. We also checked the temperature of the at the head(Engine side) and tail (exhaust side). At the head, it was 350F and tail it was 155 F. This was it, after this the mechanic said he was done with the research and wouldnt waste any more time on debugging.

Although, he had a good point - if there was anything wrong with car running either rich or lean or vaccum leaks, there would be a code. Without any code set(after recently fixing P1457) and the only code as P0420 - he asked me to order a cat. I have ordered a magaflow direct fit cat for $200 on amazon.

next week, i am going to have it installed and update on whether the codes returns or not.


thank you for all your help, i genuinely appreciate it

 

The O2 sensor on the cat is supposed to be steady, so either the problem is THAT O2 sensor, or the CAT.

As for a code being thrown when there is a vacuum leak, that doesn always happen.

but for now, see how the CAT replacement goes and hope you can post the results.

 

Yeah, I realized the problem MAY BE the rear O2 sensor. It was much clearer after reading this link -http://troubleshootmyvehicle.com/honda/2.2L-2.3L/how-to-test-code-P0420-page-1

I am kind of disappointed that on the first pass, I had to change the front O2 sensor without understanding the basics. Give the car has 128K miles, I figure it is still a good idea to get both the sensors changed.

I have ordered a rear O2 Denso sensor from Amazon, and I will be putting this in with the cat next week.

will update the progress here

 

I picked up 6 sets of stainless steel 3/8 x 1 1/2 bolts and nuts on the way to the mechanic. As suspected, the bolt heads were badly rusted and oddly enough, my mechanic decided to use a torch to melt the head instead of sawing its head off (hmm). In the process, of course, the eyes in the flanges on the exhaust pipe were eyes no more...once the bolts were taken out, the cat dropped to the floor. Shoot(i shd have taken a picture of what the inside looked like - the honeycomb was spotty/almost melted in a couple of places on the engine side...on the exhaust side, it looked pretty ok to me.

The direct fit magnaflow cat - did directly fit. Except the bolts did not have an eye to go in on the exhaust side. The mechanic used washers on the other side and welded them with to the flange (wonder how strong this solution is). A couple of years ago, I had this flanges separate and my honda was making the meanest sound on the road! I fear now.


I guess, I am good for inspection now. I have 35 miles and need to drive a 100 before any surprises pop up. I will update this thread.

 

- this is the magnaflow converter i ordered for $201 (now $189).

I have driven the car for 110 miles, went for reinspection yesterday and the car passed inspection. No more CEL for me...