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Discussion Starter · #1 ·
Hi folks -- seeking your collective wisdom / experience here!

My 2016 Focus RS popped the three codes (see below) and upon sending it to the dealer for a look, they reported back that I need to replace my cat yet again. First one went out around 30,000 miles and now this one goes out at 60,000 (I'd detecting a pattern here). Head gasket was replaced a while back so should be no issues there.

Any ideas what could be causing this? My car is modified with the following:
  • Mountune Intercooler
  • Mountune Intake
  • Mountune Charge Pipe
  • Mountune Short Shift
I've been running the stock tune for the majority of the time since I got the first cat replacement but I'll very occasionally throw on the custom tune I got from Randy of Tuneworks (then Mountune). Otherwise, I mostly drive it with a stock map. In fact, I just got a new map from Randy that I don't want to begin calibrating until I solve this issue. Some suggested getting a sports cat, but I don't wind up ruining that too if something else is going on.

Any and all insights are SUPER appreciated.

Thank you!!

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So, I track a lot in my other car, and I go through a lot of catalysts, and I've debugged these problems ad-nauseum.

Fundamentally, what kills catalysts is heat. Heat comes one of two ways. One way is when you are running too rich, and the catalyst burns up the hydrocarbons, getting hot in the process. The other way is when you are running too lean, which makes for very hot exhaust gases, which overheat the catalyst. In our cars, the too-rich condition is more likely, as too-lean would also damage the turbo real fast, and the tunes go rich at high power and WOT to avoid detonation.

I would log your pre-cat and post-cat O2 sensors over a long drive and first eliminate bad/fouled sensors as the cause of the code. The pre-cat O2 sensor should oscillate between high and low. The post-cat O2 sensor should look like an averaged version of the pre-cat sensor. Here's an older post of mine on how to diagnose sensors (P0420 simple?).

Assuming your cat is bad, start logging short term and long term fuel trims. They should both be reasonably close to zero. If they're large values, it means something is amiss with what your car thinks about A/F ratios. In my case, I was losing catalysts because of a post-cat air leak which was introducing oxygen ahead of the O2 sensor, leading the car to think it's running to lean, richening up the mixture. Other causes are something being amiss about MAF/MAP, making the ECU think it's getting more air than it really is, and making it run rich.

Good luck!
 

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I would agree with the above. But I would also state your more likely to kill the cat with a stick tune than with Randy’s.
Why do you say that? OEM's tend to compromise the tune for cat longevity and fuel efficiency, which is why aftermarket tuners can find extra power, however, that always comes at some cost. From what I've seen over many years tweaking various cars, tunes generally harm cats, but I'm not familiar with Randy's at all, so I'm curious what it does.
 

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Mainly because a tune from him is more than likely perfect t not a one size fits all RS’s.
It would be the most efficient for your car not mine and your car. Less file in the cat means less chance to build heat as it lites in the cat.
 

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Discussion Starter · #9 ·
So, I track a lot in my other car, and I go through a lot of catalysts, and I've debugged these problems ad-nauseum.

Fundamentally, what kills catalysts is heat. Heat comes one of two ways. One way is when you are running too rich, and the catalyst burns up the hydrocarbons, getting hot in the process. The other way is when you are running too lean, which makes for very hot exhaust gases, which overheat the catalyst. In our cars, the too-rich condition is more likely, as too-lean would also damage the turbo real fast, and the tunes go rich at high power and WOT to avoid detonation.

I would log your pre-cat and post-cat O2 sensors over a long drive and first eliminate bad/fouled sensors as the cause of the code. The pre-cat O2 sensor should oscillate between high and low. The post-cat O2 sensor should look like an averaged version of the pre-cat sensor. Here's an older post of mine on how to diagnose sensors (P0420 simple?).

Assuming your cat is bad, start logging short term and long term fuel trims. They should both be reasonably close to zero. If they're large values, it means something is amiss with what your car thinks about A/F ratios. In my case, I was losing catalysts because of a post-cat air leak which was introducing oxygen ahead of the O2 sensor, leading the car to think it's running to lean, richening up the mixture. Other causes are something being amiss about MAF/MAP, making the ECU think it's getting more air than it really is, and making it run rich.

Good luck!
How incredibly helpful, THANK YOU so much for that. Ford came back saying both upstream and downstream O2 sensors need to be replaced, in addition to my cat needing to be replaced. I asked them to check the Evap Purge Valve, which could be causing me to be running too rich... but they said it's fine. They also found my neutral position sensor needs to be replaced, although that doesn't effect the cat. That one makes sense because my auto start/stop hasn't been working for a very long time. Thank you again!
 

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Discussion Starter · #10 ·
You sure it’s not just an o2 sensor?
Could be part of it! Ford came back saying both downstream and upstream O2 sensors need to be replaced. We'll find out!
 

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Discussion Starter · #11 ·
I would agree with the above. But I would also state your more likely to kill the cat with a stick tune than with Randy’s.
Interesting! I always thought a stock tune would be optimized to be more conservative, but then again I do have all those Mountune bolt-ons (intercooler, intake, charge pipe)... could that be throwing off the ECU?
 

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Just go catless, no more problems.
If he were trailing the car to tracks and not street driving that is an option here. But for any street driven car in the US that is not a legal option.
 

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I just had my second cat replaced (under emissions warranty) after P0420 code at 71k miles. First cat went out around 30k miles. Car is stock apart from intake and exhaust. I drive 75% of the time in sport mode.
 

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Discussion Starter · #14 ·
I just had my second cat replaced (under emissions warranty) after P0420 code at 71k miles. First cat went out around 30k miles. Car is stock apart from intake and exhaust. I drive 75% of the time in sport mode.
Sounds like we're experiencing the same issue. I'm having both O2 sensors replaced, as well as Evap Purge Valve. Hopefully that solves SOMETHING. I'll report back.
 

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Discussion Starter · #15 ·
If he were trailing the car to tracks and not street driving that is an option here. But for any street driven car in the US that is not a legal option.
Ha, yep -- basically. Not an option for a street-driven car in California!
 

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Discussion Starter · #17 ·
Ha, I forgot I also had my Evap Purge Valve replaced. I have built a love hate relationship with my car.
I have built the very same... ha, why do we put up with this?! If the RS wasn't as relatively limited as it is, I'd have sold it by now. Plan is to hold on as long as possible, but we'll see.
 

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Ha, yep -- basically. Not an option for a street-driven car in California!
Not a legal option anywhere in the US that I'm aware of. Just California is best at enforcement of the laws regarding emission requirements.
 
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