Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum

Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum (
-   General Technical Chat (
-   -   Diagnosing P0420 ? (

PaulK1970 02-15-2014 12:05 PM

Diagnosing P0420 ?
I recently acquired a 2003 ZX3 with 143,000 miles on it. It has a few little problems and I've been learning a lot reading threads on the forum. All of the problems seem to be pretty common and I'm real pleased to have such a great resource as these forums.

I've been reading about the P0420 code and these vehicles. Some have suggested that misfires can throw the code, but my vehicle runs very smoothly without misfiring.

Others have suggested that the catalytic converters rarely go in these vehicles. I've also seen suggestions that the original computers were flashed with too sensitive a trip for this code and perhaps I should have the computer re-flashed.

I have not seen (might have missed it) a straightforward diagnostic approach to figuring out what to do next to diagnose this code.

Looking forward to suggestions. Thanks.

smtjpj 02-15-2014 06:16 PM

P0420 most often a evaporative system leak,,,,start by thoroughly inspecting all fuel and vent lines from the fuel tank forward and under hood. Code indicates a evap leak!!

BC_ZX3 02-15-2014 08:35 PM

"P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
Indicates Bank 1 catalyst system efficiency is below the acceptable threshold
Possible causes:
Use of leaded fuel
Damaged HO2S
Malfunctioning ECT sensor
High fuel pressure
Damaged exhaust manifold
Damaged catalytic converter
Oil contamination
Cylinder misfiring
Downstream HO2S wires improperly connected
Damaged exhaust system pipe
Damaged muffler/tailpipe assembly
Retarded spark timing

Compare HO2S upstream and downstream switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set."

imo, there are really only two causes for the code if the car is running well and there are no obvious problems as suggested above; the catalytic converter is well and truly done or the rear O2 sensor itself is failing (or there is a rear O2 sensor wiring fault. The rear O2 is relatively exposed to road/weather.) With your accumulated mielage, either possibility is valid.

O2 sensors are relatively cheap. Catalytic converters are not.

PaulK1970 02-16-2014 09:28 AM

That's just it, a lot of things can trigger that code it seems and I certainly don't want to replace the cat if it is not the cause of the code.

I was reading articles on Eastern Catalytics site, and they talk about air intake leaks causing this code. My ZX3 had a broken air filter box (when I got the car) and I just replaced it, and I'm curious to see if this solves the problem, but I'm not sure why it would. I don't really understand why leaky air intake should cause the code, I thought that even filtered air is pretty much unrestricted, so sucking a little unfiltered air, I'm not sure why this should matter unless it is causing the computer to add extra fuel which somehow triggers this code.

The other thing I don't get is why a bad downstream O2 Sensor would cause this code rather than triggering its specific O2 Sensor code.

Another question I have is that I was looking at direct fit replacement Catalytic Converters and I see some that specify they are a direct fit for my engine with an automatic transmission. My ZX3 is a stick, and I'm wondering if there is any difference in fitting a replacement cat, or why does Eastern specify that it is a direct fit for the automatic. I'm planning to call them and try to find out, but perhaps someone here already knows.


sykozis 02-16-2014 10:22 AM

If the downstream O2 sensor is still switching, it won't produce an O2 code because the PCM doesn't see a fault in the O2 sensor. Instead, it determines the fault to be with the catalytic converter.

As BC_ZX3 said, if you have the ability to do so, monitor the switching of the upstream and downstream O2 sensors.

If they appear to be switching at a relatively similar rate, it's most likely you have either a faulty catalytic converter or faulty downstream O2 sensor. More often than not, replacing the downstream O2 sensor will fix the issue.

mikebontoft 02-16-2014 10:36 AM

you will either need to scope the sensor or find a scan tool that will show you sensor waveform patterns...

WHEN COLD, the pre-cat (B1S1) and post-cat (B1S2) sensor waveforms will be almost exactly the same in both frequency and voltage swings (amplitude is probably the right word for that).

After driving around and getting the cat hot enough to "fire off" the waveforms will change. B1S1 will still be relatively the same as it was cold, but B1S2 will be at a somewhat more steady voltage (NO voltage swings). it won't look anything like the other waveforms.

you can rule out the rear sensor by that method. if the rear is doing something weird compared to the front during cold running, it's likely a sensor fault. If after driving, the waveforms are still fairly similar, the cat is not operating correctly.

All times are GMT -5. The time now is 12:40 AM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.1
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.