hey guys, i have been getting a p0420 code off and on for the last 3 months, and even after changing my cats(went from maggie spun high flows to 94106's) and im still getting it. well today i did the gotts O2 mod, and noticed that my drivers side rear O2 sensor had a BUNCH more soot on it than the passenger side rear O2, and am wondering if the code was telling me that something else is wrong? Leaky injector? any other ideas, or am i just overthinking this?

Adam M

 

After pulling the code and seeing BANK-1 my initial thought is vacuum problem. One thing is for sure with aftermarket add-ons eventually comes codes. My first step would be to check over the entire vacuum systems, time to get some of your favorite fluids (starting or brake) to spray around your engine compartment and listen for changes in your engines idle. The high fuel pressure as shown below is more than likely your problem if there is a vacuum leak, your mileage will probably dip by a mile or so and of course your O2 sensors will be reading rich. This all happens due to the ECM(CPU) attempting to balance the system back out as best as it knows how in certain driving conditions, but in doing so it tosses a code during certain conditions that may require more fuel or a rapid decline in fuel demand.

P0420
Catalyst system efficiency below threshold (bank 1)
Detailed Description
Indicates Bank 1 catalyst system efficiency is below the acceptable threshold.
Possible causes :-
# Use of leaded fuel.
# Damaged HO2S.
# Malfunctioning ECT.
# 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.

Diagnostic Aids
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.

 

ok ill check for vacuum leaks when i get her back tomorrow....makes sense going along with my random high idle....especially on a cold startup

 

i checked her out today, and didnt find any vacuum leaks, ill check again tom when i have more light. any ideas?

 

Vacuum leaks for a 420 ? Why ?

Search the engines forum for advice, you'll learn about the 420/430 pretty quick. Search is an awesome feature on this site and that info is everywhere.

 

i did search, no answers i was lookin for. and goin thru 2 different sets of cats(maggie spun cats, and now 94106's) and im still getting p0420 code(only p0420, no p0430, ever), so i dont think that its the cats, its more something on bank1(drivers side?) thinkin maybe the O2 sensor is goin out or something else is runnin bad.

 

If you are only getting a p0420 code with no other codes, and you have replaced the cat, replace the o2 sensor.
The fuel trim is constantly switching between rich and lean. When the post-cat o2 doesn't reflect the proper switching in its readings, it trips the p0420 code. (or p0430 depending on the bank.) Typically it's a bad cat. If the cat has been replaced I would look at the o2 sensor. It would be best to hook up a scan tool to read the live data, but if that's not an option I personally would throw a sensor at it.

 

ok, which side is bank 1? passenger or driver side? if its driver ill check my egr fitting for leaks on the LT's, then it looks like its sensor time(the rear are original at 118k....) ill see if i can get my hands on a scan tool.

 

Bank 1 is the same side that has cylinder #1.

 

I just figured if you searched, you wouldn't be checking the Vac lines. Every possibility has been posted. I know, I had them, posted them, fixed them .

 

The switch rate of the rear sensors is compared to the switch on the front. It follows that if the catalyst is efficient it will be removing HC and HO and the rear sensor will show a slow switch rate compared with the front. By dividing the number of downstream switches with those of the upstream gives a catalyst switch ratio, and a typical threshold is 75%, above which a DTC will be recorded.

It should be noted that faulty HO2S sensors do not produce the DTCs P0420 and P0430. The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive.

 

Your description of the workings of the system is right on. Your second paragraph may be correct in theory, however, I have seen an o2 sensor set that code (in a Dodge.) I spent the better part of the last decade as a mechanic. Every p0420/p0430 I have seen has been a faulty cat, except for one. On that one I exhausted all possibilities before finally replacing the post cat o2 sensor, and that fixed it.