2001 Honda Odyssey Check Engine Code Cat Converter recurring P0420

I have a 2001 Honda Odyssey with 90000 miles. Lat last year around Oct/Nov time frame, check engine light came on. I took my vehicle to a local Auto Zone store and they scanned the code and told me it was P0420 - bad catalytic converter.

I replaced the cat converter with an after market model, however after 3/4 months (after driving 2000 to 3000 miles), check engine light came on with the same code. The mechanic changed the cat converter and put a new (after market model). Again after 3/4 months (2000 to 3000 miles) check engine light came on again. And this time the mechanic replaced the cat converter and also oxygen sensor. Now after 4 months or so the check engine light is on the vehicle again. The mechanic is now saying that the problem is not with the cat converter and he is saying there is a lot of abnormal deposit on the vehicle. When he scanned again he found another issue that read something like “high voltage” and this time the guy thought may be due to short circuit (per code) the fuel injector is sending high volume of fuel and this is causing excessive fuel to go into the cylinder and hence high volume of emission. I replaced the fuse box again and after another 100 miles or, the problem is reappearing.

Has any body else seen similar issue? any possible suggestion? thank you for your time.

If the vehicle keeps damaging cats, there’s a problem with the way the engine is running.


First question: how’s your gas mileage? Has it dropped since the cat problem started? If so, that could indicate the engine is in fact getting too much gas. But if not, that explanation may be suspect.

If the engine is getting too much gas, one suspect might be a bad coolant temp sensor. If the coolant temp sensor fails to tell the computer when the engine warms up, the computer will continue to run the engine in a fuel-rich condition instead of switching to a leaner mixture after warm-up. This usually results in losing several miles per gallon off your normal fuel efficiency.

Another possibility is the oxygen sensor itself is getting contamined from silicate in the coolant, if the engine is burning coolant (which coukd only happen if the head gasket was leaking).

Just my opinion here, but this is crazy. How did your mechanic come to the conclusion that your car needed a new catalytic converter?

Fault code P0420 does not mean that your car needs a new catalyst. It indicates that your catalytic converter is operating below the minimum efficiency standards established by the federal government. That doesn’t mean you just replace the converter. Poor catalyst efficiency can be caused (or not caused but reported by) a faulty downstream oxygen sensor, a faulty upstream oxygen sensor, a malfunctioning air injection system, poor engine operation, a leak in an exhaust pipe, a fault in the ignition or fuel injection system, etc. I think you get the point.

Fault codes don’t tell us what part to replace. They tell us which system is not operating properly and give us a starting point to begin our systems testing.

By the way, a proper diagnosis will require much more equipment than a generic code reader like those used by AutoZone.

@ jesmed:

MPG is averaging around 18 /20. Last time I checked it game me 272 miles on one full gas tank (15 gallon??) (30% highway, 70% city driving).

@asemaster: Thank you I will request the mechanic to consider these ideas.

This question just came up a week ago…and it keeps coming up from time to time.

The problem may NOT be the cats…but one or more O2 sensors. More times then not…it’s a bad O2 sensor instead of a bad cat.

I have a similar problem with my 2005 Odyssey which has over 200,000 miles. Took it to the Honda dealer and they say it’s the Cat. converter, does not provide a printout of the codes or anything. Sorry but I do not trust this dealer from my last experience where they tried to charge me double for a condenser.

  1. Does it matter if you use an off market cat vs OEM?
  2. What should I check/replace in what sequence to identify the root of the issue.
  3. How many cat and O2 sensors are there on this model, the reader talks about Bank 1, is there another bank?
  4. If there are multiple banks do I need to change the same things on both banks?

Appreciate your insight and smarts.


@Vonnie, you do not need to take this car to a dealer. It’s not special or mysterious. It’s just a vehicle pretty much like any other vehicle. The number of special and mysterious things that a dealer is best at is small to none.

See all of the comments above, especially asemaster’s account of what can trigger a P0420 code - other than a problem with the catalytic converter.

Here is a good write-up as well: http://www.obd-codes.com/p0420 The way you wrote about it, it sounds like you’re going to do the checking? If so you’d probably end up needing access to a scantool as its the only way to get a look at what the O2 sensors are doing.

Being a V6 you should have a sensor for each bank of cylinders and one after the cat.

Thanks for the feedback. Should I go buy a scantool, or is there somewhere I can take it to get it checked?

Where is the best place to buy the O2 sensors if they are in factwhat is bad?

If you have a laptop and are comfortable using one you can get an OBD 2 hookup cable for not very much money. There are also free OBD 2 softwares, some that would come with the cable. Then you can add capabilities with more software but that tends to get pricey. Just search online. The basic ones check and clear codes and allow you to get a live, running look at many of the basic sensor readings including O2 sensors.

Or you pay a shop - usually about an hour’s labor. (I have an OBDLink laptop cable that came with free software and cost me about what I’d pay for an hour’s labor. If you tinker with cars it comes in very handy).

But a bunch of the stuff you also can’t check with a scantool - e.g. the search for exhaust leaks.

As mentioned above, there’s a lot of different problems that can result in this same symptom and code. Until all of them have been ruled out by doing the proper testing, usually the cat wouldn’t be replaced. Because the problem often – “usually” may be the better word – isn’t the cat. For example, if a fuel injector were sticking slightly open (leaking in other words, like when your kitchen faucet wears out and starts to drip) when it should be closed, that would send unburned gas through the combustion chamber and into the cat. And that would clog and ruin the cat over time. It is very possible for a mechanic to test for leaking fuel injectors, and the other potential culprits. That’s what needs to be done here. This is a situation where the OP should choose a mechanic who specializes in Hondas and has the Honda specific scan tool. Ask the mechanic if they can check the “fuel trim” parameters. If so that’s a good sign.