What causes catalytic converter to fail

My 2011 mustang with 94k miles was getting a P0420 code and no other codes. The only symptoms were slight loss of acceleration and power. No misfires though. Took it to mechanic today. Used the scanner and we couldn’t find any obvious faults. Fuel trims were good, oxygen sensors were fine (computer said no fault detected for both o2 sensors) and no obvious culprits for this p0420 code.

We tried doing fuel system cleaning service with seafoam and then reset the code and i’ve driven 50-60 miles since then but I can still feel the loss of power/hesitation when accelerating. The code hasn’t come back yet but pretty sure it will since the car feels exactly the same.

I’m starting to think the catalytic converter is clogged if the o2 sensors are fine. Wouldn’t that be the only other possibility for p0420?

My question is; Why would the catalytic converter fail after only 94k miles? Did my spark plugs at 88k or so. Typically cats don’t just die out with age unless they’re overheated or have a rich/lean condition which I don’t have either of. I am thinking I should just replace the cat but I know if I don’t diagnose the issue that caused it to fail in the first place, the new one will fail again. I’m just not sure where to start. I can rule out a rich or lean condition since fuel trims were normal and spark plugs since I did them 6000 miles ago.

Any thoughts?

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One item that causes the cat to lose efficiency is oil burning. How many miles after an oil change do you need to add a quart?


Doesn’t burn any oil. The engine’s in great shape in that regard. Something that’s also worth noting is that i noticed the p0420 and loss of power a day after i installed a new MPT tune. I suppose it is possible for the new tune to need adjusting but MPT is extremely reputable in the mustang community and I only have basic bolt ons and these guys have written thousands of tunes for the exact same car as mine with the same mods so i highly doubt it was the tune that caused this…i’m just stumped

@Mustangman what say you about this “tune” thing?

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A Google of the P 0420 code has a good list of probable causes. I know the next question: Which basic bolt on mods? After reading the probable causes you will see why internet guessing takes a distant second place to diagnosis, which should take almost an hour.

Here’s an idea…

“Reverse” or reset that “tune” that you did, and see if your problem goes away.


What other mods have been done to this Mustang (GT or V6?) beyond the tune?

It is possible the downstream O2 sensors are lazy. Not bad enough to throw an O2 code but enough for a 420.

The code may not come back. It might have been a learning issue of the new tune on this older engine.

As for the performance, I would put the car back to the factory tune, assuming you own a programming tool, to see if the performance returns. Then re-download the tune. Sometimes they don’t load correctly even though you think it does. That has happened to me before. The other possibility is a corrupted tune file from MPT.

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I returned the tune to stock but in terms of the loss of power the car drives the same so i’m not sure if the tune damaged the hardware so resetting it to stock doesn’t matter. I might just replace the cc and hope it doesn’t go bad again since there’s no reason why it should have went bad in the first place

It’s a 3.7 and the only other mods are an intake, roush axleback exhaust, 3.73 gears and bbk shorty headers. Stock cats and o2 sensors

Did you dyno the car? To check power levels and AF ratio over rpm?

Is this totally “butt-dyno” or have you used any tools to measure this? Stopwatch 0-60 mph or OBD2 connected app that measures this or HP and torque?

If the cat was marginal, and it could be at 94K, the tune’s richer condition at wide open throttle may have finished it off and clogged it up a bit. It may make sense to install a temporary pressure gauge (0 to 10 psi, no bigger) into the exhaust in one of the forward O2 sensor ports and do a full throttle run up in rpm to see if the pressure rises above 1-2 psi. That would confirm a clogged cat.

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Butt dyno but it’s an extremely noticable power loss. Today i replaced the passengers side cat and o2 sensor but that didn’t seem to help at all in terms of fixing the problem. Car still has loss of power and will make coughing/sputtering noise on deceleration.

If all fuel trims ratios are normal the only other culprit i can think of is a blockage in the other cat (even though there was only the code for one side). I was thinking about doing it tomorrow but wanted to check here first to see if anyone thought there could be another culprit before I spend $400 to do the other cat

the computer might take a while to relearn everything you did. how far have you driven it?

What other mods on the car do you have? I assume you’re running canned tune, and not a legit dyno tune specific to your car.

Driven it maybe 50 miles since replacing the bank 1 cat today, i don’t want to drive it too much with noticeable lack of power and with the coughing/sputtering on the deceleration.

i have roush axlebacks and shorty headers so my exhaust is plenty loud which is good because i can hear the problem better…i can hear the sluggishness in the exhaust note which sounds different than before this problem in addition to actually feeling the loss of power

Correct me if i’m wrong but backfiring/sputtering on deceleration means unburned fuel is going into the exhaust…and unburned fuel going into the exhaust means a rich condition.

But when I checked the long and short term fuel trims they were fine and according to those, the car isn’t running lean or rich.

Another guess is that a piece of the broken converter i replaced could be stuck somewhere further down the exhaust but unlikely

It’s possible the modifications to the exhaust system may be confusing the drivetrain computer programming for the fuel mixture. That could cause it to think the mixture is correct, when it is actually too rich. The computer infers whether the mixture is rich or lean from the O2 sensor readings. But the O2 sensor readings could be affected by exhaust system modifications. The computer has no direct and accurate measure of the amount of raw fuel in the exhaust; i.e. it has no HC sensor.

Its also possible there was rich operation and/or misfiring before the service you did at 88 K. That service cured it, but some damage to the cat had already occurred.

Frequent rapid accelerations and high speed driving can shorten the cat’s life.

I was running the roush axlebacks for 3 years with no problems and the shorty headers for a week with no problems. I hadn’t messed with the stock cats or sensors so it just seems unlikely they’d be malfunctioning because of an exhaust mod i’ve done

That is probably what caused the P0420 fault.

Very small exhaust leaks will allow air/oxygen into the exhaust stream resulting in a catalyst efficiency fault.


I’m not hearing a ticking noise which would indicate an exhaust leak though? Also wouldn’t that cause a lean code as well

Exhaust leaks may cause the car to run a little rich by fooling the O2 sensor. It reads the excess oxygen, adds fuel, the sensor is happy but you get more unburned fuel in the exhaust that then pops and crackles on overrun. Plus it fools the car into a P0420. Leaking exhaust is one of the causes of a P0420

They don’t always tick when leaking. It can be covered up with other noises. The leak could easily occur a month later after a gasket blows out, or a header warps just a tiny bit from heay cycling.

This discussion path is leading me to be conviced you have a header leak.