'96 Civic - Help with "Check Engine Light" Scan Code

About 6 months ago the "Check Engine Light" in my Civic came on for
the first time. I reset the ECU (by momentarily pulling the
Backup/Hazard fuse) and the light stayed out for at least a couple of
months. Since then however, the light has come back on a couple of
times (with many weeks and miles in between). After the latest
incident, I had the ECU code read at Autozone. This is the only info
that was available:

"P0420 Catalyst System Efficiency Below Threshold (Bank 1)"

Here are some clues: There has been no noticeable driveability or gas
mileage degradation. Also, whenever the light has illuminated, it has
always occurred a minute or two after exiting a highway and traveling
slowly (20-30mph) on secondary roads. The car has about 165K miles.

Is it probable that the car has a slowly failing catalytic converter or
could one of the oxygen sensors possibly cause this error code as well?
Though I'd like to drive the car another year or so, I'm certainly
not inclined to replace the converter ($$). Is my risk from not making
any repair at this point limited to possibly not passing an emissions
test?

Any comments or suggestions would be appreciated.

 

Hi

Hope this helps.


DTC P0420: Catalytic System Efficiency Below Threshold. Basically it's
saying the catalytic converter is not working properly. But before we
go and replace it, we need to check it.

First clear all the codes from the Powertrain Control Module (PCM)
thusly.

Turn the ignition switch OFF , and remove the No.13 CLOCK BACKUP fuse
(7.5 A) (A) from the passenger's under-dash fuse/relay box (B) for 10
seconds.

Now, start the engine and hold the engine at 3,000 rpm with no load, in
Park or neutral, until the radiator fan comes on.

Test-drive 40 to 55 mph for approximately two minutes. Then decelerate
for at least 3 seconds with the throttle completely closed. Then reduce
the vehicle speed to 35 mph. If the light comes back on, you will need
to replace the catalytic converter.

Frank
www.NGKPartFinder.com - Best site to find all NGK products for your
vehicle.

 

Thanks, Frank.

I will try your test; The procedure seems designed to produce very
similar conditions to those that have caused the light to come on in
the past: highway driving, then stop momentarily after exiting highway,
and then slow driving (~30mph).

What may happen if your test is positive and I continue to drive the
car without replacing the catalytic converter?

Marc

 

wrote in

Other than failing an official emissions test or the MIL illuminating?
Nothing.


What Marc's test is doing is accelerating the rich/lean test your ECM does
every trip anyway. Check for air leaks first.