P0420: Why is my cat possibly dead?

herniated · Dec 8, 2017

Hi everyone,

I have a P0420 code on my 2011 Sport 4x4 and I believe my bank 1 cat is dead:

rattling sounds coming from the area
voltage waveforms that match each other on O2 sensors 1 and 2.

Can you help diagnose if it is dead and why?

History:

I got the check engine light and P0420 code. 135000 km on the odometer.
Changed my air filter, 47000 km since last replacement. Very dirty from dusty gravel roads.
Checked my O2 sensor voltage waveforms on bank 1 and downstream sensor 2 was following upstream sensor 1.
Changed my O2 sensors to new NTK sensors. Old Ford sensors had sooty black tips. No change to voltage waveforms, still following each other.
Cleaned MAF sensor with no residue electronic cleaner. No change.
Long term fuel trims are bank 1: 0-2% and bank 2: 2-4%. What does this even mean?

Ideas? Solutions?

Thanks!

RonD · Dec 8, 2017

Welcome to TRS

P0420 really means the Downstream O2 sensor voltage is changing too much.

Downstream O2 is the one found behind Cat

The 2 Upstream O2 sensors(assume V6 engine) are used by the Computer to adjust the air/fuel mix on the fly.
O2 sensors see Oxygen, not fuel, O2 sensor voltage range is .1 to .9 volts
(you may have wide-band O2's in 2011, which have 0-5volt range, but works the same)

.1 volt is Rich, or too little oxygen
.9volt is Lean, or too much oxygen

Computer adds fuel, via fuel injectors, to try and keep Upstream O2 voltages between .2 and .8volts, and this voltages changes a few times a second, so very fast "on the fly" adjustments, and thats Fuel Trims, short term fuel trim(STFT) is the Open time for fuel injectors on that bank of the engine, base on RPM and Air Flow data available to computer.
It varies from -10 to +10 in normal operation
Long term fuel trim(LTFT) is the average for each bank over the "long term", -5 to +5 is normal
If either STFT or LTFT get to -15 or +15 for any length of time computer will set a Lean or Rich Code

Downstream O2 is after the Cat, so "sees" Cleaned the exhaust, it should have a steady .7 or higher voltage, no big ups and downs like upstream O2s
P0420 means Downstream O2 sensor is reporting incorrect voltage

Short Video here: https://www.youtube.com/watch?v=KGpMzdEO5KY

If engine was ever run Lean or Rich for any length of time it will burn out Cats.

You could just have a lose/burnt wire on Downstream O2 sensor

herniated · Dec 8, 2017

Thanks for the welcome.

Yes, both sensor voltages on bank 1 are switching quickly between 0.1 and 0.8, which is why I think my cat is dead. I don't think I have a wiring issue because that's what I was getting before and after replacing my O2 sensors.

It appears that my bank 1 cat may be dead. I don't think it's because my original O2 sensors were failing at the time, causing the system to run overly rich or lean, because the old and new O2 sensors show the same waveforms.

BTW, bank 2 is showing healthy waveforms, with upstream switching nicely and downstream at a steady 0.7.

RonD · Dec 9, 2017

OK, some confusion here

Bank 1 Sensor 1(B1S1) is the Upstream sensor for bank 1

Bank 1 sensor 2(B1S2), is the downstream sensor, after the cats.

Bank 2 sensor 1(B2S1), is the upstream sensor for bank 2

Bank 2 sensor 2(B2S2) would be a downstream sensor but usually only found on Dual exhaust systems.
Most vehicles just have the one B1S2 downstream sensor

So any O2 sensor "2" should be steady higher volts

herniated · Dec 9, 2017

Yes, I can understand why what I wrote might be hard to keep track of and if you look back at what I was writing I was trying to relate that:

B1S1 and B1S2 are following each other, which is why I think my cat is dead. For comparison and to indicate that I understand how the waveforms should look, B2S1 is switching nicely and B2S2 is at a steady 0.7 V, showing a healthy cat.

Still want to know why B1 cat may have died.

RonD · Dec 9, 2017

Cats are killed by Lean condition, they get too hot and melt inside
Cats are killed by Rich condition, they get a coating of unburned fuel inside and then it is ignited by a spark, which rips up the insides.

And long term they rust from the inside out, like all exhaust components, or they run out of chemicals

herniated · Dec 10, 2017

Would like to know what the underlying issue is before I change the cat and possibly kill another.

Here's some sensor data:

O2 sensors, long term fuel trim, MAF and EGR.

RonD · Dec 10, 2017

I looked at 2011 Ranger Sports and don't see extra tail pipe for Dual Exhaust
And it is not listed in any data files as factory dual exhaust or even an option.

So do you have Dual exhaust, have you looked under the truck?

Nothing in the log files indicate and problem with upstream O2s or anything that would cause an issue with Cats.

Wondering if your Forscan reader is setup wrong or misreading Ford PIDs(packet IDs)

O2S12 should be the ONLY downstream O2 sensor

O2S22 would also be a down stream sensor but only with Dual exhaust, and it is reading correctly

Yes the match between O2S11 and S12 is like the Cat is not even there, hollowed out
So if you do have Dual Exhaust(4 O2s) then that Cat is bad, manufacturing defect maybe.
If you have no history on the vehicle it could have run Lean or Rich for extended period on that Bank and you are the one paying the bill for that now.
But I see nothing that would indicate a new Cat would suffer the same fate

herniated · Dec 10, 2017

Thanks for sticking with this thread. I have 2 cats, 4 sensors, 1 tailpipe.

You don't see anything that would cause any issue for the cats? Are you determining that from the fuel trim data? Any other relevant data you can think of that I should try to pull?

Bought it at 80000 km, had really good inspection. Have been maintaining it since, now at 137000 km. Changed plugs and wires at 112000 km, how do these plugs look?

Had a recent inspection with good results at 125000 km, they said it's all good and running well. So at this time I'm looking at ordering a new cat and bolting it in. Any opinions on Eastern Catalytic vs Magnaflow? The Ford OE cat/y-pipe is super expensive at $1500 from the dealer, compared to $300 to $600 online for Eastern and Magnaflow.

P0420: Why is my cat possibly dead?

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