GregoryD

CEL on recently. Code reader shows P0420. Checked gas cap - on tight. Came on in December and dealer told me catalytic converter needed to be replaced. When I asked what would cause it to fail I was told a) cats just fail & b) can't really tell until replaced. Doesn't make sense to me and I don't want to spend $800 on a new cat just to have it fail again. There seem to be many factors that can contribute to P0420. Please help me understand.

GregoryD

Cleared the code. Hasn't come back yet, but I recall it takes a number of run cycles before it triggers CEL so could still be present. Do cats just fail, or is there something wrong upstream, like fuel injectors that are not regulating fuel properly?

GregoryD

120,000 miles. Sole owner. All regular maintenance and recalls done as they came up.

ashtons300zx

Yes the 3.5L motors had issues with the converter and yes they do just randomly go out. I have an 05 mdx and the CEL PO420 has been on since 2 days after I bought it. Since the dealer just erased the code and sold the car to me. That was at 55k miles. It now has 142k miles on it and still has the same converter.

There are a couple things that could be causing the code that arnt converter related but if your engine is running fine then non of these probably apply to yours.

1. These motors have issues with the exhaust valve lash closing up and causing the valve to remain slightly open and sending unburnt fuel into the converter plugging it and causing the o2's to read funky throwing the code. However, there are symptoms of this that you would know if this is the issues. The motor will idle rough/misfire and with a scanner you can look at the fuel trim and vacuum and see that you have a messed up fuel trim and low vac readings as well as funky o2 readings.

I just did my valve lash at 141k miles and only a couple of intake valves were slightly loose and a couple exhaust valves slightly tight but nothing that would cause an issue and my engine is pulling great vac and idles smooth as silk. So I figured this wasnt an issue but did it anyways.

2. o2 sensors improperly working. When I first go the code I also had a code for o2 sensor bank 1 sensor 2 slow response and replaced it. The cat inefficient code went away for about 2 weeks then came back on and stayed on. If your o2 sensors are going bad they can cause the code because they will think there is an improper reading between sensor 1 and 2 causing the inefficient code. Again you can check this with a scanner that has the capability to read engine data. All my other o2's are operating fine so I know its down to an issue with the converter.

3. Using regular instead of premium fuel. When you use regular unleaded fuel instead of premium, the ecu puts more fuel into each cycle to combat pinging/detonation. This is why you will always get worse mileage with regular than premium on these motors. Ive tested this. When you use regular the extra fuel and exhaust gases can cause cat failure and plug it up.

4. This seems to be the most common and that is they are just crappy converters and just fail. This seems to be very prevalent on the ULEV models which came out in high emissions requirement areas such as California and New York which is where mine originally came from. You can tell if yours is on the front window glass, there will be a sticker that says ULEV compliant emissions vehicle. Mine is a ULEV. On the ULEV programmed ecu's they are much more picky on how the converter is working vs regular ones. So while there may not really be a major issue with the converter and if your car is running fine and seeing the same or close gas mileage as it did before the code then I really wouldnt worry about it unless you live in an area that tests this.

These are great engines and vehicles and love mine to death but they are just plagued with these stupid converter problems. I bought an aftermarket converter to put on but havent done so yet. I do the work myself so yeah I dont have to fork over money for someone else to do it. One thing i noticed right away with the aftermarket cat is that the converter is much smaller than the oem so im also skeptical of putting it on.

Anyways, if you live somewhere where there is no testing then id say ignore it. It wont do any damage to the engine. If you start to see poor mileage and loss of power then by all means please replace it as at that point its probably completely plugged up. But like I said, almost 90k miles put on mine with the cel on and no problems.

GregoryD

Thank you for walking me through the possibilities! What frustrated me the most is the dealer's service manager jumped to replacing the catalytic converter without even mentioning dealing with any of the potential causes.

It is a ULEV version, so I guess Premium fuel would be in order. The exhaust has always seemed a little fuel-ly to me. I'll try a few tanks and see what happens. BTW - my local mechanic (for routine service - the nearest Honda dealer's about 25 miles away) recommended the same thing: reset & forget, since emissions aren't tested in this area.

lazlong

^^ You can erase and forget but, your MPG's will go down significantly over time. If it was me I'd look at the O2 sensors first (cheapest and easiest to fix). Also, once the cat goes bad, you can always hollow the cat and use the O2 sensor bypass trick since you have no emissions standards.

GregoryD

Good point about the decrease in MPG. Would that be from the catylitic converter plugging up? According to my local mechanic the O2 sensors tested OK. I guess I could replace them and the cat, but I'm also wondering if something is out of whack in the ECM, or if the fuel injectors are worn. If too much fuel's getting into the engine then changing the cat and the O2 sensors is just a temporary fix since they'll burn up again.

Accordman1

No Honda requires super unleaded gas except the S2000 so don't waste your money on premium gas. However, I would say try to use better gas like Exxon or Shell. First, check the O2 sensors to make sure they are working. Next, check the O2 sensor reading using a scanner after the car reach operating temperature. The primary O2 should be switching fast between 1 and 5V and the secondary O2 sensor should be fairly stable at 4-5V. Next, take the temperature of the cat at the inlet and the outlet using an infrared thermo scanner. The outlet should be at least 100F high than the inlet. If the cat checks out, the problem is elsewhere in the engine.