Just acquired a 2005 EX Sedan 2.4L with Manual Transmission. 140,000 miles and runs quite well. Smooth accelleration, smooth idle...maybe slightly less power than my same generation 2003 Accord with 240,000 miles.

I ran the trouble codes and got P0420, Catalyst System Below Threshold (Bank 1). I understand that is the O2 sensor below the Cat. According to the previous owner, that sensor was recently replaced by a mechanic.

The owner disclosed that it was having issues passing emissions so I'm prepared to spend some money making it right again, but I wanted to get some ideas from HAF experts on how to diagnose what the real problem may be before I go on a parts replacement marathon. Basically, if the Cat has failed, I want to know why before I replace it with another one that I will ruin in a few months.

Here's what I've noticed so far:
1.) It does not seem to burn oil; no smoke on startup or accelleration, dipstick stays constant.

2.) Compared to my 2003 with same drive train, which has a clean tail pipe, I can lift a coat of soot off the inside of 2005.

3.) Gas mileage on my 2003 averages about 31MPG; I get 27 in the 2005.

4.) I've cleared the codes twice since purchase and the MIL lights about 75-100 miles after clearing.

5.) I checked the codes before the MIL lit last time and got a "Pending P0420 Error", "Catalytic Converter Not Ready", and "Evaporative System Not Ready" message.

The following is a list of Reported Fixes I got from O'Reilly Auto Parts, but I'd like to diagnose and fix the real problem before completing this punch list of repairs:

Replace Catalytic Converter (is it possible to check if it's really bad?)
Perform Catalytic Converter Burn-Off Procedure (sounds great, but what is that?)
Program Powertrain Control Module
Replace Air Fuel Ratio Sensor
Replace Heated O2 Sensor Bank 1 Sensor 2
Perform Emission Monitor Drive Cycle (What is that?)
Replace Exhaust Pipe (Not yet)
Replace O2 Sensors

Thank you to anyone who looks into this.

 

long story short, diagnostics lead to replacing the TWC (three way catalyst) or you may have poor fuel quality. next time you fill up, use a name brand gas, chevron, 76 or other...stay away from independents and arco. add a can of Seafoam to the tank. drive 20 miles or so and then clear the code. let us know what happens next.

 

try to clean the cat by running some lacquer thinner in the gas tank. Look for videos by Scotty kilmer

 

If that still doesn't work replace the cat and recycle the old one at converterguys.com they gave me two thirds the price of the new cat back

 

Turns out it was the catalytic converter. Just spent the entire day replacing it after trying seafoam and a burn-off according to Scott Kilmer. Just got back from theOBDII readiness drive and no trouble codes...no pending codes either! The old one was plugged and has a lot of loose material rattling around inside. Now I will try to find a recycler. Thanks for the tip on converterguys.com!

 

you're not done...you need to find the cause of the convertor failure...they just don't fail on there own, especially at a 140k.

 

my '99 failed at 150K, no other problems

 

I've been fretting about the possibility of recurrence. Looking at the converter, I'm wondering if the PO already fixed the problem without knowing it. There is evidence that they spent some money trying to get it to pass emmissions recently and I'm guessing they put it up for sale because they couldn't figure it out, or didn't want to spend the money to replace the cat. It is obvious they, or their "family mechanic" tried many things; it has new spark plugs, new gas cap, new radiator cap, new O2 sensors...I think they got tired of dumping money into it.

I've had Hondas go 400 plus thousand miles on the original cat so I know there is a likely reason for the failure (although, I'm not ruling out that it just went bad). What evidence can I look for in the old cat to figure out a possible cause?

When I had the exhaust system pulled apart I did not notice oily residue or soot buildup in the system pre-cat; It was clean. However, After the cat, there is plenty of soot on the walls of the pipe. The cat itself has a lot of loose material clanking around inside and I can't see light through it like I can the new one. The honeycomb material on the front and back of the old cat seems intact and there is no sign of warping from overheating. I've only driven it about 500 miles total since buying it, but there doesn't seem to be signs of oil consumption.
What else can I check? Air flow sensor? What other diagnostics can be done to be sure it's running it's best?

 

Check Long Term Fuel Trim (LTFT) w/ an OBD2 scanner. If it's near zero (not sure how near zero but probably < +/- 5%) and not changing, things are fine.

If LTFT is running higher than -5 to -10% or more, then excess fuel is being added and ECU is attempting to compensate.

good luck

 

broken-up substrate in the convertor is indicative of running too lean for a period of time (overheated) ...may have been solved by the previous owner. my 03 has some sooty residue at the tailpipe and has run just about all zeros every smog check since i've had it. you are probably fine, just keep an eye on things