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P030X Random Misfire & Limp Mode Questionaire and Discussion Thread

What solution was administered to address your P0300/Limp Mode Issues

  • Cam Sensor(s) replaced

    Votes: 0 0.0%
  • Wiring Harness replaced

    Votes: 0 0.0%

  • Total voters
Hello all,
Last night I did a track night and experienced a misfire code.

A little background....... My CEL light stays on because I do not have the cats on (codes P0420 and P0430). I just leave it on no big deal. what concerns me is when I am really pushing the car hard it starts running rough and the CEL begins flashing. I noticed then then Bank 1 was running lean (20:1 or around there) while Bank 2 was normal (14:1).

In between sessions I looked quickly at the codes and saw a misfire code. I was in a hurry to get back out there so I quickly cleared them and went and added 5 gallons of 100 octane fuel thinking it could have been poor fuel.

Next run had it again. It sounds terrible and runs rough and then would straighten up. When I got home I read the codes again and there was not any misfire codes..... ? just the usual Catalyst codes.

I have read about the O ring fix and crankshaft sensor relearn. Is the poor rough running associated with this problem? Or am I perhaps dealing with something different? I was running the track key.

Thank you, Keith
O-ring and relearn are cheap and easy fixes. Start there. Also look for build up in the sensor when you pull it for the o-ring.
Do you have a tune??

I did have a custom tune but just recently went back to the stock tune/track key tune.

Is the crankshaft position sensor something I can do myself? I thought there was some sort of dealer reflash needed? Thanks!


Because racecar.
Arizona, USA
I'm not sure of an official test for the O2 sensors--but theoretically you could swap them (left-to-right) and see if the code re-occurs and if the Lean bank is on the same side as before or not. If it's on the same side as before, the sensor is OK. If the lean bank follows the swapped O2 sensor, the sensor may be the culprit.

Mad Hatter

Gotta go Faster
Santiago, Chile
Seems O2 sensores need to be replaced quite often. I have gone through 2 in 50.000km. Maybe LT headers are a reason? Not having them working well leads to scary outcomes....

Every time I remove a tranny for any reason (more often then I ever imagined!) I do a crank relearn with a sct X4.

Sent from my SM-G900M using Tapatalk
Sorry for bringing up old thread, after almost 5 years(only 35kmiles) of owning my 13' gt auto engine check light started flashing today with moderate acceleration.

Got a p0300, i know theres a tsb out for o ring and a relearn. But is there anyway i can get away with just replacing the o ring without the relearn or just disconnect my battery for few hours? has anyone tried it without relearn and worked?

I didnt experience any power loss of that sort, is the misfire code a glitch? if its just a annoying flashing light i think i can live with it.

I have alot of bad experiences with dealership when it comes to repairs and i rather do it myself.

it seems procal tuner has the ability to do the misfire relearn, i might do that while gettin on ford racing tune since im still on stock tune.
Last edited:

302 Hi Pro

Boss 302 - Racing Legend to Modern Muscle Car
Are you thinking about this Ford Technical Service Bulletin?

If not, (as this is for MT6 5.0 Mustangs built prior to November 30, 2012), you can visit LMR’s website and use their TSB|Recall library. Very handy resource.

If your repair is covered under a TSB and if it states: “Using IDS, perform the Misfire Monitor Neutral Profile Correction procedure“, then I would not try to bypass this step.

Good luck in diagnosing your CEL cause and please keep us updated on your fix.
I bought a used 2015 Mustang GT three years ago. It had 20,000+ miles on it. It ran fine for a year and a half. I started getting random/multiple misfire codes every other time I drove it to work. It's never went into a limp mode, but it's annoying seeing the light on almost every time I drive the car. I'm at 53,000 miles now. I probably need to check and see if I have any warranty left. I don't track my car, and most of the driving is highway.
I had some bad misfiring codes solved with new o2 sensors. I think I'm running into the issue again and won't hesitate to swap them out to at least rule out that issue.

In the process, I was driven mad trying to chase the issue. I cooked a magnaflow cat that ended up clogging one bank of the exhaust. Rule out the o2 sensors

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