Code P0171 System too Lean. (not the PCV hose) - Page 4 - Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum
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Old 07-03-2019, 12:13 PM   #31
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Another thing I thought of. My exhaust resonator is leaking. I think it's the resonator. It's the one just before the rear axle. Long, round cylindrical. I know an exhaust leak can cause an issue like this but this is a leak near the rear of the vehicle. I wouldn't think that far back would cause the O2 sensor to misread.
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Old 07-04-2019, 07:53 AM   #32
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I drove another 30 miles home and the light is still off. STFT is +-0% and LTFT is 10%

O2 sensors are reading good.

So who know's. All I did was unplug the MAF sensor, run it for 10 mintues and plug it back in. We'll see what the future brings.
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Old 07-04-2019, 10:12 AM   #33
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Id say if it does come on again it's likely the MAF sensor.

Cal

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Old 07-04-2019, 11:03 AM   #34
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Quote:
Originally Posted by Stangfire View Post
I drove another 30 miles home and the light is still off. STFT is +-0% and LTFT is 10%

O2 sensors are reading good.

So who know's. All I did was unplug the MAF sensor, run it for 10 mintues and plug it back in. We'll see what the future brings.


The only thing I would check now is that all the diagnostic monitors are complete. Your scan tool will tell if they are/arent.

Paul
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Old 08-22-2019, 07:56 AM   #35
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Hi,

Came across this while hunting my own p0171.

Tried the MAF unplug and it cleared nearly everything up for me, but when unplugged my car now shifts super hard. Did this happen with yours?

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Old 08-22-2019, 11:13 AM   #36
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Quote:
Originally Posted by pizzaboy192 View Post
Hi,

Came across this while hunting my own p0171.

Tried the MAF unplug and it cleared nearly everything up for me, but when unplugged my car now shifts super hard. Did this happen with yours?

Sent from my Moto Z2 using FF Mobile

That did not happen to me. I have a 5 speed. I only had it unplugged for 20 minutes of driving. then plugged it back in and cleared the CEL. it's stayed off for now.
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Old 08-22-2019, 11:44 AM   #37
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Originally Posted by Stangfire View Post
That did not happen to me. I have a 5 speed. I only had it unplugged for 20 minutes of driving. then plugged it back in and cleared the CEL. it's stayed off for now.
Good to know. Turns out Ford uses the other sensors in the MAF to determine shift points in the autos.

Found an eBay seller (looks like a dealership) selling genuine unused Ford MAF sensors for $30 instead of $200, so I bought one. I appreciate all the details you provided in your posts because it helped me figure out I probably have a similar issue (and I discovered a block heater while I was unplugging the MAF)

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Old 08-22-2019, 01:54 PM   #38
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Quote:
Originally Posted by pizzaboy192 View Post
Good to know. Turns out Ford uses the other sensors in the MAF to determine shift points in the autos.

Found an eBay seller (looks like a dealership) selling genuine unused Ford MAF sensors for $30 instead of $200, so I bought one. I appreciate all the details you provided in your posts because it helped me figure out I probably have a similar issue (and I discovered a block heater while I was unplugging the MAF)

Sent from my Moto Z2 using FF Mobile
Good luck. You don't have a leaking PCV hose? That's what everyone told me was my issue. I changed it and it was perfectly fine. I also changed it without removing the intake manifold when everyone said it has to be removed.
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Old 08-22-2019, 02:57 PM   #39
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Good luck. You don't have a leaking PCV hose? That's what everyone told me was my issue. I changed it and it was perfectly fine. I also changed it without removing the intake manifold when everyone said it has to be removed.
I haven't replaced mine yet because I don't have the time to, but all signs point to it not being the pcv hose (steady vac pressure, no hissing, etc) plus even when I'm driving if I give it a shot of starting fluid right to where that hose is, it doesn't react.

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