Newly Purchased 18 GT Went Limp.

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Someone will be along shortly to offer advice.

If you had TPS sensors mis-matching (there are two for safety reasons), you would have a code for that. Ditto if you had a MAF problem or transmission problem.

I think you have a couple of options here.

1) Take the vehicle back to the Nissan dealer and raise a fuss. It seems fairly obvious to me they knew it had issues (which may have been why it was traded in) and sent you on your way after just clearing codes. You probably won't get very far, especially if the vehicle was sold "as is". Note this is only option if you no longer want the vehicle and at best you might get trade-in value on a trade-in.

I am not nearly as familiar with the S550 as prior generations, but I wouldn't think a catalyst efficiency code would put the vehicle in limp mode. (I know it does not on the S197) Does the vehicle even have functional UPSTREAM O2 sensors??? Non-functional upstream O2 sensors might put the vehicle in limp mode. Others with more expertise can clarify.

3a) Your third option is to get yourself a handheld tuner (e.g., SCT X4) and "data log" your vehicle. A data log records things like fuel trims, O2 sensor voltages, MAF readings, and more. This sounds intimidating, but in reality is not too bad. Plug the scan tool into the OBD-II port and press "data log". Then download the file. You can post your datalog file here and some of us can interpret it using software (freeware). Alternatively, you can send the file to a tuner who can read it and send you a better tune you can flash to your ECM that corrects your problem.

3b) A variation of the above option is to get your vehicle to a reputable tuner where they can perform the above 3a) for you. I think your issue is resolvable with a tune provided you at least have UPSTREAM O2 sensors. I do not wish to violate forum policy here nor the law, but it is possible for you to obtain a tune with the rear O2 sensors disabled since you are catless. I do not condone this and you do not wish to violate laws and you will eventually need to get an inspection sticker. This is at best a temporary solution, \\ No matter what you need UPSTREAM O2 sensors to control fuel trim.

4) Your fourth option is take it to a Ford dealer or a very good shop. Unless you are planning to supercharge this vehicle, there is really no reason for LTHs. Request the factory exhaust manifolds and cat system be re-installed and the tune flashed back to OEM spec. You will definitely want to be sitting down at the point you receive the cost estimate. Yes, your warranty has been voided.

You can retain the mufflers and what not downstream of the OEM cats. This will put in a 50 state legal position too. \\\

Apologies for typos. BEST WISHES!

Rft3rd said:

Let me preface this post with the fact that this is my first experience with a Mustang or really any performance car. Yes, it is a mid-life crisis buy so, please pardon my ignorance.

I purchase a used 18 GT with 24k miles on Tuesday.

The dealer informed me it had some exhaust modifications so as I am not savvy on the ins and outs, I had a trusted friend and mustang guy take a look at it for me.

The modifications were not insignificant.

Corsa Extreme Cat-less long tube headers, Borla ATAK valved Exhaust but stock airbox (appears they had a CAI on there at one time as the sound hose was removed and the port on the airbox was uncovered, more on this later.

Sounds fantastic and the active valve features work great. That's the good stuff.

On the way back from the dealership (about 60 miles) it threw a code at me. Got it scanned and it was for catalyst malfunction (Duh). No big deal in reality but it got me thinking about whether the car was ever tuned after the headers were installed and IF so, why weren't the Cat o2 sensors deleted?

Well on the way back down to the dealership for the detail they owed me, it hit Limp mode on me while going 70. Had no idea what the hell was going on as I had never even heard of it before.

The car seemed to stick in 9th unless downshifting, would jerk when coming off acceleration, and seemed to stop at 2k rpms. and accelerating from a dead stop was terrifying. All things I see are normal for limp mode.

I limp along to the dealer and they experienced the same thing. They thought it was bad gas, I had just filled it up with mid-grade. Keynote, the salesman filled it with regular when they delivered the car and it ran fine through the entire tank so I am not too sure about that. They did check the battery as that is a common source for this too.

Looking up car symptoms on the internet is like web md saying everything is cancer.

Other possible issues could be Throttle Position Sensor, MAF sensor (which makes a little sense if the intake is sucking more air from where the sound hose used to be?) as well as transmission sensors.

They confirmed the only code on the ECU was the Catalyst code I was aware of and of course, after being off for a minute, the limp mode was reset. They did say that it appeared a tune was done on the car but could not verify what the tune was. This was a Nissan dealer and they were in communication with their Ford dealership and the Master Tech there.

They put an octane booster in the tank and sent me on my way. No issues on the 60 miles headed back home.

So long story short (too late), What is the best way to get this figured out? The car has 2 months of factory warranty left on it but I am afraid with the headers and tune, the warranty is void.

Thanks in advance and I look forward to hearing your thoughts!

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You bought someone else's problem. If you didn't know what to look for when buying a used car then you should have had your friend, a Ford dealer, or a garage check out the car before you bought it. Now you have to deal with the aftermath of not having a used car, a sports car no less, inspected first.
The dealer definitely knew that the car had an illegal catless exhaust on it and sold it to you anyway. Idk what the details of the sale were but if they sold it to you "as is" then you might not have any recourse now. Though being a Nissan dealer, I assume that there's some kind of warranty from them. If so, then they're on the hook for fixing it but most dealers will string you along until any warranty period is up and then it's even more of an uphill battle to get things fixed without an attorney getting involved.
If you have a 30 day return window or your state has a used lemon car law that applies in your case which allows you to return it then I suggest that you unload that car ASAP. The 2018 was the first year for the 10 speed auto and it has a multitude of TSBs issued for it. A TSB is a technical service bulletin or known as a manufacturer communication as the NHTSA calls it. It's not a recall; it's a known manufacturer defect that under warranty would be fixed for free but out of warranty it's at your cost or the dealer's if they have liability.
There's also 2 serious recalls regarding that trans and one other unrelated recall for a fuel line leak. Maybe they were performed and maybe they weren't but with you having trans now issues there's a good chance that you have a lot of unresolved TSB related issues with it.
With no cats there's definitely an older(now illegal) aftermarket tune on the car which means you effectively have no more Ford powertrain warranty. It would have voided out most aspects of the Ford warranty. The car should have come with the handheld tuner that the previous owner had because without it now you would have to go to a Ford dealer to get them to put the factory software back on it if you were to get the car back into compliance with cats. To get it fixed this is likely going to be necessary since it's now illegal for any shop to actively work on the exhaust and keep your car with those emissions modifications. If you have anyone work on the catless exhaust they will certainly tell you that you need to buy a catted mid pipe with O2 sensors and return the car to an emissions compliant tune. No aftermarket tunes for catless exhaust are available from any tuners either. This was completely outlawed a year or two ago.
I would immediately insist that dealer buy a catted midpipe from Ford(not an aftermarket one), get the exhaust into legal compliance, fix the air intake back to stock, and then send the car to Ford for a factory reflash of the PCM. I would also be threatening to have my attorney get involved if they don't agree. They sold you a car that was not legally compliant.
Get a descriptive receipt or work order for every time you drop off and bring that car in to be "fixed" at that dealer so you have a record. Don't accept any word of mouth stuff from the service manager.