P0420 (CEL) on, off, and on again
 

Hi folks,

The car is a 2009 Fit Sport with 206,719 miles.

The very first time check engine light (CEL) came on was January of this year 2017, days after heavy rain when I drove through a pool of water. The code was P0420. I had it cleared and no codes for 3000 miles (2 months). In March, the same code and CEL came on, also days after rain. Cleared it again.

Then nothing with CEL occurred during the 9 months afterward, even with several road trips, including to Arizona desert in the summer.

Last week on a raod trip, the CEL came on, after a rain overnight and parking outside. Driving 279 miles later, the CEL turned off on its own. But after another 158 miles, the CEL came on again. The code was P0420 and a mechanic cleared it. Today CEL lights up again.

My question is, does the P0420 definitely point to the Catalytic Converter? Can it be false-positive and something else triggered the computer?

I was told that the Cat. is about transferring heat away, rather than chemical catalytic reaction. I take care of the car and have 14 pages (8 years) of notes documenting all the services and repairs, even jotting down miles when 10% drop in the MM. I pay attention to and keep up with the car. Would like to learn.

I'd greatly appreciate any insights or advice. Many thanks in advance.

G

Here's a good read...

The Misleading Nature of the PO42O Code - Eastern Manufacturing

In most cases I've seen/heard of, the car ends up needing a new rear O2 sensor. Go with the OEM or Denso direct replacement from a reputable source if you buy one. Lot's of fakes/copies out there.

Thank you so much. Will read that.

I'm struggling with that evey shop I went, they just said replacing the cat. My gut feeling is to change the O2 sensor first. I don't know if any shop out there would check the waveform of the data from the front and back O2 sensors.

Is there anything that could have caused catalytic converter failure? Ever had a bad plug, coil, etc? If the air/fuel ratio has ever been off, the likelihood of a cat failure goes up exponentially.

Car up to date with valve adjustments and spark plug changes?

If everything else is good (no reason to believe it's ever run rich or lean), I'd probably lean toward O2 sensor degradation as the cause before actual catalytic converter failure.

As far as I can feel from the driving, I don't believe I had a bad plug or coil (how to tell?).

The last valve adjustment was at 125,425 mi (81,294 miles ago) and last change of spark plugs was 108,059 mi (98,660 miles ago). The oil life just lowered to 15% and Maintenance Minder is showing A-1-4, and I have scheduled an appointment for changing plugs and doing valve adjustment next week (plus oil change and tire rotation).

The only thing that is a mystery to me is that the mpg has dropped over time. In the first 4 or 5 years, the car could easily average 41 or 42 mpg per tank (combined freeway and surface streets). Slowly it started to drop without changing driving patterns or commutes. And now, it only does 36 to 37 mpg per tank.

When you say that the MPG is dropping, is that on the dash meter or hand calculated results after fill up?

Thanks for chiming in.

I use the figures from the dash display, just one way (consistent?) of seeing the mpg throughout the 8 years of the car.

I would not put much faith in them being accurate or consistent.

My experience with many cars is that they are neither.

I appreciate that spirit of questioning a computer display.

And that is why I ask here about the P0420 code. What condition should be corrected or check first before replacing the catalytic converter?

Great to hear input from others about their issues of P0420. Please help.

I would probably pick up an OBD2 scanner/reader if you don't have one and follow the section in the link above regarding fuel trim and O2 sensor readings.

That or take it to a qualified mechanic that will do it before just replacing the catalytic converter. Anyone can change parts, but a mechanic can diagnose the real problem.

Other option is you could buy secondary/rear O2 sensor and put it on and see what happens.

Problem with the throwing parts method is it could get costly and waste time.

Thanks again for the link. I did read the whole thing. Very instructional, it is essentially a flowchart written in prose.

I think you are right-- I may have to buy an OBD2 scanner. Have been putting off the purchase for many years. It's like, I have a multimeter in my electronic tool box. Don't use often, but when I need to check something, it is there.

Appreciate your thoughts.

I watched a vid yesterday on checking you O2 sensors with a multimeter and it looks like it can be done without a OBDII unit. I'm not familiar with any particular unit so I can't say one way or other if the unit will provide the diagnostics you need to pinpoint the problem: fore / aft O2 sensor or Catalytic converter.

Over the years I've had O2 sensors fail on various vehicles and I keep all of my vehicles to approx 150K miles but never had a cat failure.

It's just more prudent to check the O2 sensors first. Avoiding the unnecessary cost of a cat replacement could cover the cost of a OBDII tool if it provides the data you need.

Like you said, I am still leaning on first targeting the O2 sensor (I was told that in my car, only one is monitored), making sure it is not faulty. I am starting to look at which OBDII to get.

And if the catalytic converter is faulty, then I plan to replace it. BUT I want to first correct the condition that has ruined the cat., so that the new one won't suffer the same.

Next week I am taking the car to service A-1-4, spark plugs and valve adjustment, and this time I am using a different mechanic than the one who did those last time.

It is so hard to find a mechanic who is open to see beyond just the P0420 code. Many see the code and that equates to absolute order to replace the cat.-- and nothing else.