Fuel System coding for rich and lean conditions

Steven Technician Huntington Beach, California Posted   Latest  
Discussion
Driveability
Rich And Lean Codes

Fuel frustration

Fuel system coding for rich and lean conditions,

For years techs have been taught that the Sensor 1 on Banks 1 and 2 were solely responsible for fuel trim adjustments. Then around 2004 the term, “Final Fuel Trim” started to be thrown around, and although no one really put much thought into it, the world was changing.

Case in point, a mid-2000’s Subaru coding for rich mixture. Both banks S1 are in the 800-900 mV range, fuel trims in the + 20’s, most sensors were performing as expected, with the exception of the B1S2, it was parked below 200 mV. There was much confusion over the question of “How can the fuel trim go positive when the Bank 1 & 2 O2 sensors were reporting a rich condition”. Phone calls were made, tests and repairs were done, valve adjustment, compression test, replacement of sensors and more tests, some efforts made minor improvements, but the condition remained.

A fresh set of eyes appeared on scene, He propped the throttle open to about 2500 RPM and after three or so minuets the B1S2 started to move and the fuel trim started to turn around taking fuel away and started to head towards zero, the prop was removed and 20 seconds later the post cat O2 fell back below 200 mV and the trims started to add fuel. The man explained that on several car lines in the mid 2000’s changed the rules and started using Post Cat O2 sensors for primary fuel trim adjustments to get fuel economy to Federal standards or some such nonsense. The eyes of the group glazed over, he had lost them. Anyway, he said, the heater in B1S2 has expired, replace the post cat sensor. They did and the Subie straightened out and flew right.

The current belief held by some is that the post cat O2 sensor is there for the sole purpose of testing catalyst efficiency.

The up to date information available in the training materials for manufacturers disputes this belief, identifying the post Catalyst Oxygen sensor or AF sensor as the primary source for fuel trim guidance.

Has anyone else experienced this condition?

All the best, 

Steven …

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Steve Instructor
Irvine, California
Steve Default
 

Spot-on thinking. I have had a number of vehicles that had fuel trim issues only to be a deteriorated O2 sensor. Good post. Thanks Steve.

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi Steven: The short answer is yes. I've heard it explained as being similar to the "fine tuning" knob on old television sets. The S1 sensors were the equivalent to the "tuning" knob. The S2 was the fine tuning. They're often used for cold running strategy (LTFT). On a Mini, you very well may get a P0171 using an OBD-II scan tool. You can trust me when I tell you that the OEM code bears no

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Nelson Manager
Huntington Beach, California
Nelson Default
 

You are spot on Anthony, i refuse to diagnose BMWs with P codes.

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Michael Mobile Technician
Clinton, Utah
Michael Default
 

Hi Steven, Saw this on a 2004 infinity today. It is evident we can't make assumptions any more. Thank you for your write up. -Mike

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Sad to say, most of us were taught the use and operation of oxygen sensors wrong. Back in the late 80's Toyota started putting post cat oxygen sensors in some of their vehicles. That was the first rear oxygen sensor fuel trims that I am aware of. When OBD2 came along in 1996, many manufacturers would trim from the rear sensors from time to time. the sad thing was, nobody ever gave the rear trim

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Steve Instructor
Irvine, California
Steve Default
 

Albin, Toyota, Honda, some Ford's, most manufacturers that use Bosch and Continental controllers utilize the rear O2 for final adjustment. There is little OE documentation on the rear O2 being used for fuel trim until the mid-2000's, even though it was used in the mid 90's.

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Robert Diagnostician
Ballston, New York
Robert Default
 

I have seen this. I also was taught what an o2 is for and how it functions. Originally most of us were taught that the upstream sensor was to provide a feedback as to the efficiency of the combustion so fuel trim changes could be made to keep the exhaust gases clean enough for the cat to perform properly and keep the engine running well. A new way of thinking that’s work for me is that the

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Jose Technician
Woodland, California
Jose Default
 

Hello Steven, I repair smog emissions failures and have run into some late 90's Toyota sedans where this issue has been the case. Replaced rear oxygen sensor and tailpipe emissions come down to normal range. Have only encountered it a couple of times.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

I haven't experienced it personally, but I attend all the training I can and, the info has made it's way in there now. These discussions help me remember what I am taught even when I don't see it. As a non-emissions test area, I don't do near as much CEL diag as I had hoped for by now. I am actually "the diag guy" at the shop here but I can go days without using a scan tool at all.

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Brendan Owner/Technician
Browns Plains, Australia
Brendan Default
 

I have experienced this on a similar era Subaru to you. It is almost logic defying to me - - You have a superior wideband 02 sensor up front, why put such weight to the reading of a lesser narrowband sensor mounted after a cat who's efficiency is also going to make an impact on the oxygen content. It would be very interesting to get some factory data on how much weight is given to each

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Randy Analyst
Denver, Colorado
Randy Default
 

What you are describing on a mid 2000's Subaru does not match up with my experience. Can you provide an exact year make model so that we can better understand what you saw? Any data? Subaru's use several different fuel control strategies. for example, a 1997, 1998 and 1999 model will have 3 different strategies and how they fail will confuse you if you don't understand them. Around 2000 they

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi Randy: "2010 and beyond only saw fuel control get more and more complex to the point it is nearly unexplainable. I certainly can't wrap my head around all of it." From the man who can make my head hurt, that's scary! It bears repeating that we need to understand what we're working on. (I've been guilty more times than I like and will probably be so again in the future.) They're not all

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Steven Technician
Huntington Beach, California
Steven Default
 

Randy, great response! You speak to the complexity of the systems we service as well as to the absolute need to know your terrain. The details of the vehicle escape me. It was one of several I had seen over the lest weeks with similar property's. The confusion that accompanied them caused me to wonder who else might be struggling. I will look through my cases and see if I can find the

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Danny Technician
Losangeles, California
Danny Default
 

This Toyota Tundra came in with a bad bank 1 A/F sensor. It was controlling fuel via the rear O2 sensor.

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi Danny: Nice capture. I assume this showed Loop Status as CLF. I can envision more than one person who would want to put a Cat on it. Guido

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