P0171

[almartin]

2012 Sonic LS (1.8L), ~150k, had the infamous P0171 come on about a week ago. Pulled the codes (scan tool results below).

Air filter is new. Checked the air intake flex-hose for cracks but didn't see anything. (It has a generic aftermarket one as the original breathed it's last months before the recall came out.) Cleaned the MAF sensor.

 

[almartin]

Code came back the next day (scan results below). The car runs fine other than idling a touch low. No hesitation, fuel economy seems o.k. for just running around town.

Any ideas?

 

[TurtleOverlord]

30% fuel trims seem really high, probably compensating for the lean code.

can you check the voltage reading on the primary o2 sensor? it may be bad.

 

[almartin]

I'll check it.

 

[almartin]

Finally got a probe in to measure voltage. Signal bounced around 30-75 mV for a bit, then the engine changed pitch and the number would go as high as 0.7 V. Then it would settle back down. Got behind the wheel to watch the rpms when the pitch change occurred and saw that when the higher pitch happened the rpm was not changing noticeably, but the ESC/TCS light was coming on. Stayed on for 1-2 sec then out and the pitch went back to normal.

 

[orangeltz13]

Finally got a probe in to measure voltage. Signal bounced around 30-75 mV for a bit, then the engine changed pitch and the number would go as high as 0.7 V. Then it would settle back down. Got behind the wheel to watch the rpms when the pitch change occurred and saw that when the higher pitch happened the rpm was not changing noticeably, but the ESC/TCS light was coming on. Stayed on for 1-2 sec then out and the pitch went back to normal.

Have you checked your MAF sensor? My car is doing the same thing and I have the same codes. I tried using a cleaner but it didn't work. I ordered one from O'Rieley's for $116.00. I had to order it because it wasn't in stock. I also have a CAI on my and I am wondering if it is partially responsible. I am not convinced CAI systems are worth it; the theory doesn't make sense. I will reply back after I install the new MAF. I have a 2013 1.4MT Sonic with 102K miles and purchased brand new with 50 miles on it because it came from a dealership 50 miles away.

Finally got a probe in to measure voltage. Signal bounced around 30-75 mV for a bit, then the engine changed pitch and the number would go as high as 0.7 V. Then it would settle back down. Got behind the wheel to watch the rpms when the pitch change occurred and saw that when the higher pitch happened the rpm was not changing noticeably, but the ESC/TCS light was coming on. Stayed on for 1-2 sec then out and the pitch went back to normal.

A mass (air) flow sensor (MAF) is a sensor used to determine the mass flow rate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each cylinder

 

[almartin]

It looks ok, but I need to get some cleaner and go over it anyway. Won't hurt.

 

[JTee2017]

I had something similar happening on daughters 2014 1.8L. I found a vacuum leak with a cracked connector from the brake vacuum accumulator to the intake manifold if I recall. But I do not remember the error code that was generated.
In my case, the cracked connector leak would go intermittent . I found it by gently moving vacuum lines and connectors one at a time.

 

[almartin]

Cleaned the MAF, code remained.

Took off the negative battery cable for an hour then reattached...2+ weeks without a code.

 

[orangeltz13]

Cleaned the MAF, code remained.

Took off the negative battery cable for an hour then reattached...2+ weeks without a code.

Great! My issue was caused by my oil cap not being installed correctly after an oil change by the Valvoline Instant Oil Change technician.