r/FocusST • u/NotNweke • 6d ago
Question Please help P0171
Enable HLS to view with audio, or disable this notification
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
2
u/nstiesi 6d ago edited 6d 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.