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ls2gto2, ASE Certified Technician
Category: Car
Satisfied Customers: 137
Experience:  Master Certified ASE Technician, working at GM delership, 3 years training hybrid/auto/diesel
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Have a 2002 legacy and keep getting the engine warning light

Resolved Question:

have a 2002 legacy and keep getting the engine warning light on after motorway drive.
Subaru found PO171 system too lean. Changed exhaust gaskets,crankcase vent valve.
Submitted: 1 year ago.
Category: Car
Expert:  ls2gto2 replied 1 year ago.
Hello and thanks for using JustAnswer! My name is ***** ***** am a master certified technician. I will be happy to assist you with your question. Sorry for the long delay, i will be asking you a series of questions to give you the best possible help that i can. A P0171 system lean code can be set by several conditions. low fuel pressurefailed air fuel sensor, aka Oxygen sensorintake air leakMAP sensor reading incorrectly worn spark plugsfuel injectors clogged. fuel pressure regulator problemsplit vacuum hose. The most common cause for this concern is a failed air fuel (A/F) ratio sensor. Do you have a scan tool that can monitor live engine data? The (LTFT%) long term fuel trim, and oxygen sensor data values can be used to help diagnose this concern. Normal LTFT valves are normally between +or-10% If the value is higher at idle this is normally caused by a air leak. If the LTFT value is high under a load then low fuel pressure is very likely.
Customer: replied 1 year ago.
Thank you for this, I do not have any equipment but will pass this on to my Subaru service people and get back to you.
Expert:  ls2gto2 replied 1 year ago.
No problem, let me know what you find. Also i forgot to mention, The technician can disconnect the A/F ratio sensor before the catalytic converter. Drive the vehicle and monitor the rear Oxygen sensor values. If the LTFT starts to return back to normal then this is a good indication that the A/F ratio sensor pre catalytic converter will need to be replaced. If you could kindly rate my service it would be very appreciated so i can receive credit. I will continue to work with you even after rated, and get notifications about any other questions you may have. I only strive for excellent service, Thank you for being a great customer.Derek W
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