Check Engine - Code 0420

Howdy, I have a 99 Civic EX. It's got about 122,000 miles on it.

Recently the 'Check Engine' light came on, and I had AutoZone pull the
code for me.

The Lady told me.... "Bank 1... 0420.... Catalyst efficiency below
threshold."

So far, from basic searches on the web, I am assuming it's either the
catalytic converter going out, or an Oxygen sensor.

Is there any thing we can troubleshoot to help me decide which step I
should do next?

The car runs strong, drives great. I got 33.5 MPG on my last tank of
fuel, and I don't drive like grandma. If the light hadn't come on, I
would not suspect a problem. One thing I'll mention, is while at
Autozone, I did buy some fuel injector cleaner. Maybe this is
coincidence, but I filled up with gas that same day and put in the
fuel injector cleaner. Later that week the check engine light turned
off... only to come back maybe a week (and different tank of gas)
later, and it's been on ever since (a few more weeks).

Thanks gang.

 

<snip>


Three possibilities:
1) Cat is slightly crudded up with carbon and other deposits. A good hard
one or two hour's run on the highway may burn off enough of it to make the
error go away for a while.
2) Cracked exhaust manifold, or other air leak after the primary sensor,
but before the cat.
3) The cat really is degraded and in need of replacement.

If you're in the US, the cat is covered by an eight-year, 80K mile federal
emissions warranty. The warranty is transferrable, and starts the day the
car was originally purchased. You're out of warranty, unfortunately.
Aftermarket "universal fit" cats are readily available in most
states/provinces for about a couple hundred dollars installed.

 

A catalyst efficiency code can be a bad O2 sensor more likely a bad cat.
Unfortunately, the only way to really tell is to watch the sensors data
stream on a scanner and see if the downstream sensor matches the
upstream sensors voltage fluctuation. If it does, you have a bad
converter.

At home, you can do a temperature test. With the engine idling, the
pipe behind the cat should be 25-50 degrees hotter than the front,
showing the cat is working. If its not, throw an O2 at it. Thats
usually cheaper than a new cat.

Or you could take it to a local shop and pay them a diagnostic fee to
tell you exactly what it is.

Or you could take this as an excuse to buy a header, a high flow cat,
and some fancy exhaust for your civic! :grinyes:

 

Tegger and Mobil12, thanks for the answers. I'm somewhat leaning
towards being a "Parts Replacer" and just swapping out the Cat first.
I was thinking of buying one, then taking it to the local shop to have
them install it.

My car is a 127hp EX, I have no delusions that there's secretly a v8
under the hood. But Mobil12s comments are intriguing, I've no problem
putting in something more "hi-performance" than what was stock. I'm
not interested in paying much a premium for it though. What would
y'all recommend?

Thanks folks

 

I highly doubt that. OBD-II sensor monitoring is pretty tight.

It's not supposed to match. The secondary's fluctuation is far *slower*
than the primary for the simple reason that there's supposed to be very
little oxygen after the cat. A P0420 is set when the secondary flip-flops
*too fast*, which is interpreted as there being too much oxygen getting
past the cat instead of being adsorbed by it.

The ECM regularly tests the cat and secondary by commanding rich for five
seconds to deplete the cat's oxygen store, which stalls the secondary's
voltage fluctuation. The ECM then immediately commands lean for five
seconds, then times how long it takes for secondary fluctuation to resume.
If the time exceed a pre-programmed figure, the ECM assumes the cat's
oxygen storage capacity has declined, and the MIL is illuminated with
P0420. Some cars originally had overly sensitive P0420 tests. In those
cases, the automakers have ECM reflash procedures to ease the limits. It's
worth checking with your dealer to see if there's such a thing for your
car.

When O2 sensors go bad, they typically fluctuate TOO SLOWLY, not TOO FAST.

This won't tell you anything. Emissions limits are so damned low it takes
very little degradation to cause the ECM to condemn the cat. In all
likelihood your temperature test will appear to show the cat as being
perfect.

Now there's a good (expensive) idea.

 

For $89 I bought "LapLogger" which is an OBD2 interface and some fancy
software to see your data realtime trending on your PC. It will do the
O2 sensor waveform test. In my P0420 case, it was the catty, not the
O2 sensor.

Todd K.