Diagnosing a P0420 code
 

I keep getting the p0420 code that indicates either low catalyst efficiency or a faulty downstream o2 sensor. Obviously, the CAT is possibly the problem. But I suspect the CAT efficiency could be undermined by another problem, such as burned oil or unburned gas, so I want to find and fix any such issues. The car still drives fine, so I keep driving it, but I have a SMOG test due soon.

Symptoms:

The P0420 code has lit up the CEL 8-10 times but in one situation only: steady cruise, light throttle, freeway speed, after the car is pretty thoroughly warmed up (at least 8 or 10 miles of driving). It has not yet come on when doing any of my typical hypermiling, such as P&G, which involves high load and short runs.

Diagnosis (pictures!)

[I will edit this to upload pictures in a few minutes]

Blackened upstream O2 sensor:
http://ecomodder.com/forum/attachmen...1&d=1529895123
This sensor was cleaned throughly to near new appearance about 2500 miles ago.

Slightly plugged CAT honeycomb:
http://ecomodder.com/forum/attachmen...1&d=1529895126
The dark spots are probably plugged honeycomb, and across the entire surface it looked like this. I cleaned this CAT with the acid wash method in 2016 and the honeycomb was all uniform light grey and clear afterward.

Fuel fouled plug #4:
http://ecomodder.com/forum/attachmen...1&d=1529895126
All four plugs looked about the same, like this one, but this one (#4), was wet and smelled of gas, too. The plugs were all used but clean and gaped properly about 2500 miles ago.

Possible causes.

As steady cruise, I have also noticed the longterm fuel trim gets lean by a percent or three. So I have been wondering about a rich fuel condition. Obviously, the fuel fouled spark plugs suggest that.

I have wondered about a partially plugged up CAT, which obviously kinda seems to be the case in the photo.

Next steps

I already tried cleaning the O2 sensors. That solved nothing. So...

1) Test spark plugs and wires and replace the plugs if indicated.
2) Maybe wet compression test to look for oil blow-by.
3) Maybe injector test to look for leaking
4) Clean catalytic converter and O2 sensors.

YOUR THOUGHTS?

P0420 is almost always a dead cat. The description of your test conditions when the code sets are part of the drive cycle. That is when the cat efficiency test runs. What happens is the lean rich swings match on both sensors and this tells the pcm that the cat is not working. If the cat was working the two sensors would not have the same readings at the same time. I would try cleaning the cat that is between the two sensors.

What I might try is get it tested, if it passes by a lot it probably is the O2 sensor.
If not the code is correct and catalyst is below efficiency threshold, try to clean it.
Converters have been using a lot of palladium over the years, palladium is near a 15 year high, so you don't want to buy one if you can help it.

Buy a spark plug anti fouler and drill it out to 1/2 inch and install it on the downstream sensor and just be done with it. Worry about it next time you have to smog it.

Thanks for the reference to the testing cycle. It never occurred to me that the testing might be intermittent and not constant. I have been thinking it is constant and that it sets a CEL when a dip below say 80 or 85% efficiency is detected. But this website says the threshold is 50%. So according to the website, my car sometimes detects below 50% efficiency across two drive cycles and other times does not for weeks. What could cause intermittent efficiency drops like that? Maybe the gasoline of spark plug #4 is a hint?

I would rather identify the problem in advance, not least because it appears there are other problems besides the CAT. What do you guys make of the gasoline on spark plug #4? Seems the spark is not firing completely sometimes, which might mean the gasoline/air mix is being sent straight into the CAT. If that happens when the ECU is running a test of the CAT and boosting the amount of fuel in the mix, then a gasoline/air mix burning in the CAT could possibly send O2 across the downstream sensor sometimes, no? That might trick the ECU into thinking the CAT has failed when actually spark is weak. No? Look at my picture of the CAT surface in my car. That's pretty clean: no melting, no cracking, no black spots, no flaky powder. Just some off-coloration.

As I mentioned in the OP, I have a SMOG test soon. Gotta get it done in July.

Fuel trim reading 1-3% lean wouldn't raise my eyebrow at all. Have you looked at the spark plug again since driving it more? Wet and gasoline smell could well be because the engine was shut off on the intake stroke.

P0420, in my experience, rarely lies. I'm sorry I forget what instrumentation you have, if you have an Ultragauge I know you can watch O2 sensor voltages. The data resolution sucks but should be passable for seeing if your downstream sensor is switching or holding fairly steady. Other instruments may have this function as well, I just don't know.

Do check the spark, just to be on the safe side, but if you had a serious enough spark issue to burn up the cat, I'd think you would see it on the fuel trims, and under 3% correction for your long terms is great; the rule of thumb I've heard is that more than a 10% correction is when you need to start worrying.

An interesting thing then is that short term trims will read very lean, sometimes way over -10% and often under high steady load (climbing a 1.5 mile steep hill on my commute) I have seen short term trims of -26%. My long term trim might be deceptive because P&G has always produced modestly positive longterm trim, such as +0.78 or +1.56. So what happens on the freeway now that I have taller tires and am steady cruising at light throttle is the longterm trim gradually moves into negative territory. If I had a longer ride it might get -10% or higher eventually. That make sense? If true, my car has been possibly feeding unburned gas into the CAT for a VERY long time.

I resistance tested my ignition wires just now on a lunch break: the #4 with the unburned gas on the plug has oddly low resistance in the wire. I got a bunch of reading near zero on the 20k scale. See attached notes:

http://ecomodder.com/forum/attachmen...1&d=1529954275

Number 4 wire is faulty, I gotta think.

[EDIT: AND I TOTALLY WILL CHECK THE OBD2 DATA OUTPUT FOR THE DOWNSTREAM SENSOR. THX.]

If you have a misfire or leaky injector that could clog up the converter if ran long enough.
If you fix the mission fire and give the car an Italian tune up it may go away on its own.

If one spark plug wires is not like the others, replace it.
Normally they develop really high resistance, not low.

Hmm ok, strange the long terms aren't adjusting (they usually follow short terms fairly quickly) but with short term that far out, then yes I would definitely want to check out fuel and spark. If you have an INCANDESCENT test light (say NO to LED for this test), ground the claw and hold the other side out from the boot while cranking or running. It's a quick and dirty test but you want probably at least a 1/2" of spark (preferably 3/4") reaching out to your test light from the bottom of wherever the conductor is buried in that boot, and with the resistance of the incandescent bulb you both put extra resistance in the circuit and avoid damage to the ignition and/or any computer drivers.

Low resistance is strange, but remember that you're checking that wire unloaded, and unloaded circuits are strange. Put said incandescent test light in-line with your multimeter and see what the resistance does, compare to the other wires.

Can you see both O2 sensor patterns at the same time? When they match it trips the P0420 code. There needs to be a lag in the second pattern for the code to go away. This is best seen on a graphing scanner like a Modis or a laptop with the correct software. We have mostly Fords at work and we have the same computer programs the dealers have. This has been great being able to flash computers and diagnose drivability issues.