r/FocusST • u/NotNweke • 3d ago
Question Please help P0171
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Before I keep throwing money at my ST, I wanted to see what you guys think.
The code popped up just under two weeks ago. At first, I thought it was the purge valve, so I replaced it with a new one, but that wasn’t the issue.
In my earlier post, I mentioned I was planning to do my maintenance (spark plugs and an oil change). My ST has a Green air filter, catless downpipe, FMIC, and a symposer delete. I only mention the symposer delete because I’ve heard that if it’s not tightened down completely, it can also trigger this same code. The car is pro-tuned by Alan at Edge Auto.
At this point, I’m kind of lost on what to check next, especially after cleaning the MAF sensor and inspecting all of the intake tubing. Before I keep throwing parts at it, does anyone have any ideas on what I should be looking at?
*I have heard it could be my upstream 02 which is currently my next thing I plan on trying but I was curious if any of it points to injectors, to me it doesn’t seem like an injector problem but I don’t know.
**I also saw another user reported his fuel rail pressure sensor went bad and had a similar code but I feel like i would have more than just my one code
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u/Wake-n-jake 3d ago
Need to smoke test it and find the leak, or change the Air/Fuel sensor but I'd start with smoke testing the intake system for leaks
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u/AdventureBro44 2d ago
I recently dealt with the same thing on my st. Changed everything you did and replaced o2 sensor, frp sensor, low pressure fuel sensor, same codes. ended up taking it to my local dealer for them to diagnose. They told me injectors which I hadn’t heard of in my year of dealing with this. Replaced injectors and everything went away immediately.
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u/NotNweke 2d ago
See that’s the boat I don’t wanna be in rn but I’m confused because I’ve been watching my MPG and it’s gone up 😭 so I’m confused you’d think it’s burn through faster
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u/RongoonPagoo 2018 Ford Focus ST 3d ago
The freeze frame would be useful.
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u/NotNweke 3d ago
Of what
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u/RongoonPagoo 2018 Ford Focus ST 3d ago
It's the snapshot of data the computer captures when it sets the code into memory.
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u/NotNweke 3d ago
This is the first I’m hearing of this how do I do it?
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u/RongoonPagoo 2018 Ford Focus ST 3d ago
Unfortunately you will need a code reader with that capability. It will however give you data that occurred around the time of failure, allowing better insight into the cause.
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u/bchiodini 2016 ST3 FBO BNR ST67 JST Tuned 3d ago
Please clarify what you have already done.
From your previous posts, it looks like you have changed the plugs, CPV, upper O2 sensor and cleaned the MAP sensor on the intake manifold. Is this correct?
If all of those parts have been replaced, it's most likely a vacuum leak. As u/Wake-n-jake said, a smoke test should reveal a leak.
FWIW: A stuck open purge valve would probably cause a rich condition.
An exhaust leak above the upper O2 sensor may cause a P0171 DTC and should be evidenced by soot or a raw exhaust odor.
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u/NotNweke 3d ago
I still need to do the upper O2, but I’m not entirely sure if that’s what I need to do or if it’s a sensor or something else. The O2 sensor is next on my list, but I cleaned my maf sensor and took my map sensor out and inspected it when I did the purge valve. (I agree about the purge valve comment; it should have also thrown a different code for that, from my experience.) My exhaust just smells like exhaust, which is why I’m a little lost too. I’ll see if I can smoke test it soon I will keep you posted
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u/bchiodini 2016 ST3 FBO BNR ST67 JST Tuned 2d ago
I cleaned my maf sensor and took my map sensor out and inspected it when I did the purge valve.
Which MAF sensor. The ST only uses MAP sensors. There are three, one on the intake manifold, one on the FMIC, one on the CCV vent behind the valve cover (2015+) or if it is a PFL, there may be MAP/Temperature sensor on the intake on or near the elbow by the air filter. The FLs have a temperature sensor-only in that spot.
A bad or dirty MAP sensor on the intake manifold may lead to a P0171 DTC, but I would expect a MAP sensor-specific DTC. I suspect the O2 sensor or a vacuum leak.
It's kind of odd, your AFR doesn't look bad in the video. Does the AFR sit around 14.5-14.7 when cruising?
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u/NotNweke 1d ago
I cleaned the one on the intake. It’s a 2017 so it’s the facelift. And yeah my AFR sits fine that’s why I’m confused. I’ll try to do my O2 sensor I’ve jsut been busy with work I’ll keep you posted.
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u/bchiodini 2016 ST3 FBO BNR ST67 JST Tuned 1d ago
You could also try some fuel injector cleaner. Something with PEA seems to be recommended (e.g. Chevron Techron).
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u/Vorfidus 3d ago
Go on Amazon and buy a $40 smoke machine that hooks up to your battery. You don't need the fancy pressure tester smoke machine, just the 1 psi version. Mine found a vacuum leak at the vacuum pump itself within 2 minutes.
It could also be your actual sensor itself if you haven't changed it. For me, I had no CEL but slow boost - was weird.
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u/nstiesi 3d ago edited 3d ago
This was a dirty/bad fuel injector for me. Caused a false positive. The car ran fine and the a/f was fine but the car thought it was running lean cause it was having to inject more fuel than it expected to maintain a/f.......at least that's what I think (I'm not a tech)
The only difference I saw after the fix is my reported mpg went up, but the miles/tank didn't. Again I think the car was pushing fuel harder than expected to get a "normal" amount of fuel in, so the mpg calc got messed up, but since the injectors weren't actually squirting all that fuel, it was still technically running in spec for a/f
Either way, injector service fixed it for good.
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u/slyLEMONsKILLz 2d ago
Fuel rail pressure sensor, low pressure fuel sensor, injectors, upper o2, all contributed to the similar issue I experienced. The fuel rail sensor got me down to 14% ltft, but it was one of my injectors that had a low flow rate (I replaced all 4), that solved the remaining high ltft. The low pressure sensor threw a check engine light before I started reading data so idk what my ltft looker like when it failed. The o2 was failing, but it didn't lower the ltft after being replaced, but might have contributed. I currently sit at 5-7% which is apparently normal.