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Frontier Nissan Frontier/HB/Truck Discussion Forum

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Old 11-26-2019, 10:47 AM
nellison nellison is offline
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2008 Frontier Crew Cab
 
Join Date: Jan 2019
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Question Long Term Fuel Trim

Recently replaced both Cats in my 2008 nissan frontier 170k as well as the 2 downstream 02 sensors. Now my gas mileage is horrible around 14~15mpg when I was getting around 22mpg. FixId shows that I have P0174 bank 2 - live data shows a Long term fuel trim bank 2 @ 18.75%! 02 #2 sensor shows .71volts and 02 #6 shows .94volts. NO MECHANIC, so I am learning. Do the front 02 sensors need changed out or are the downstream 02 sensors bad that I just replaced with new ones? Any help would be greatly appreciated!!!
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Old 11-28-2019, 04:07 AM
smj999smj's Avatar
smj999smj smj999smj is offline
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2006 Pathfinder
2003 Frontier King Cab
2003 Frontier KC SVE 4x4
 
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The rear oxygen sensors do not effect engine management; they are only catalyst monitoring sensors used by the ECM to make sure the upstream converter is efficient, so it's not them. The front oxygen sensors are actually air/fuel ratio sensors, which operate a little differently and can be hard to determine if they are working properly outside of the ECM triggering a code. They are expensive and usually last a long time, but should be replaced any time the upstream converter is being replaced and most converter warranties specify that proof of replacement is required to enforce the converter's warranty. P0174 occurs when the system is too lean, which means the ECM will counter will adding more fuel. Since it is bank 2, which is the driver's side, the first thing I would check for is an exhaust leak on that side at the flange gasket between the exhaust manifold and the upstream converter. I would also look at the manifold flange for signs of cracks where a look could occur. There have been a few reported cases of cracks developing at that particular area on these vehicles and perhaps while working at replacing the converter, it caused this crack to open a little wider? If you had a bank 2 catalyst code prior to the replacement, it may have caused that, as well. A disconnected vacuum hose or dirty/bad MAS can also cause lean system codes, but usually you'll see codes for both banks, not just one.
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